diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 9ec88e0be3e..19ddbcdac04 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -17892,6 +17892,9 @@ namespace ts {
                     }
                     if (requestedExternalEmitHelpers & NodeFlags.HasAsyncFunctions) {
                         verifyHelperSymbol(exports, "__awaiter", SymbolFlags.Value);
+                        if (languageVersion < ScriptTarget.ES6) {
+                            verifyHelperSymbol(exports, "__generator", SymbolFlags.Value);
+                        }
                     }
                 }
             }
diff --git a/src/compiler/emitter.ts b/src/compiler/emitter.ts
index 99b0535da47..09c785a24e3 100644
--- a/src/compiler/emitter.ts
+++ b/src/compiler/emitter.ts
@@ -73,51 +73,40 @@ var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, ge
 
         const generatorHelper = `
 var __generator = (this && this.__generator) || function (body) {
-    var _ = { label: 0, sent: function() { if (sent[0] === 1 /*throw*/) throw sent[1]; return sent[1]; }, trys: [], stack: [] }, sent;
+    var _ = { label: 0, sent: function() { if (sent[0] === 1) throw sent[1]; return sent[1]; }, trys: [], stack: [] }, sent, f;
     function step(op) {
-        if (_.flag) throw new TypeError("Generator is already executing.");
-        while (true) {
+        if (f) throw new TypeError("Generator is already executing.");
+        while (1) {
             if (_.done) switch (op[0]) {
-                case 0 /*next*/: return { value: void 0, done: true };
-                case 1 /*throw*/: case 6 /*catch*/: throw op[1];
-                case 2 /*return*/: return { value: op[1], done: true };
+                case 0: return { value: void 0, done: true };
+                case 1: case 6: throw op[1];
+                case 2: return { value: op[1], done: true };
             }
             try {
-                switch (_.flag = true, op[0]) {
-                    case 0 /*next*/: case 1 /*throw*/: sent = op; break;
-                    case 4 /*yield*/: return _.label++, { value: op[1], done: false };
-                    case 7 /*endfinally*/: op = _.stack.pop(), _.trys.pop(); continue;
+                switch (f = 1, op[0]) {
+                    case 0: case 1: sent = op; break;
+                    case 4: return _.label++, { value: op[1], done: false };
+                    case 7: op = _.stack.pop(), _.trys.pop(); continue;
                     default:
                         var r = _.trys.length > 0 && _.trys[_.trys.length - 1];
-                        if (!r && (op[0] === 1 /*throw*/ || op[0] === 6 /*catch*/ || op[0] === 2 /*return*/)) {
-                            _.done = true;
-                            continue;
-                        }
-                        if (op[0] === 3 /*break*/ && (!r || (op[1] > r[0] && op[1] < r[3]))) {
-                            _.label = op[1];
-                        }
-                        else if (op[0] === 6 /*catch*/ && r && _.label < r[1]) {
-                            _.label = r[1], sent = op;
-                        }
-                        else if (r && _.label < r[2]) {
-                            _.label = r[2], _.stack.push(op);
-                        }
-                        else {
-                            if (r[2]) _.stack.pop();
-                            _.trys.pop();
-                            continue;
-                        }
+                        if (!r && (op[0] === 6 || op[0] === 2)) { _.done = 1; continue; }
+                        if (op[0] === 3 && (!r || (op[1] > r[0] && op[1] < r[3]))) { _.label = op[1]; break; }
+                        if (op[0] === 6 && _.label < r[1]) { _.label = r[1], sent = op; break; }
+                        if (r && _.label < r[2]) { _.label = r[2], _.stack.push(op); break; }
+                        if (r[2]) { _.stack.pop(); }
+                        _.trys.pop();
+                        continue;
                 }
                 op = body(_);
             }
-            catch (e) { op = [6 /*catch*/, e]; }
-            finally { _.flag = false, sent = void 0; }
+            catch (e) { op = [6, e]; }
+            finally { f = 0, sent = void 0; }
         }
     }
     return {
-        next: function (v) { return step([0 /*next*/, v]); },
-        "throw": function (v) { return step([1 /*throw*/, v]); },
-        "return": function (v) { return step([2 /*return*/, v]); }
+        next: function (v) { return step([0, v]); },
+        "throw": function (v) { return step([1, v]); },
+        "return": function (v) { return step([2, v]); }
     };
 };`;
 
diff --git a/src/compiler/transformers/generators.ts b/src/compiler/transformers/generators.ts
index 133ed01f091..e11804bd467 100644
--- a/src/compiler/transformers/generators.ts
+++ b/src/compiler/transformers/generators.ts
@@ -137,11 +137,7 @@ namespace ts {
         Endfinally              // Marks the end of a `finally` block
     }
 
-    type OperationArguments = [Label]
-                            | [Label, Expression]
-                            | [Statement]
-                            | [Expression]
-                            | [Expression, Expression];
+    type OperationArguments = [Label] | [Label, Expression] | [Statement] | [Expression] | [Expression, Expression];
 
     // whether a generated code block is opening or closing at the current operation for a FunctionBuilder
     const enum BlockAction {
@@ -245,6 +241,7 @@ namespace ts {
         const previousOnSubstituteNode = context.onSubstituteNode;
         context.onSubstituteNode = onSubstituteNode;
 
+        let currentSourceFile: SourceFile;
         let renamedCatchVariables: Map<boolean>;
         let renamedCatchVariableDeclarations: Map<Identifier>;
 
@@ -297,7 +294,9 @@ namespace ts {
 
         function transformSourceFile(node: SourceFile) {
             if (node.transformFlags & TransformFlags.ContainsGenerator) {
-                return visitEachChild(node, visitor, context);
+                currentSourceFile = node;
+                node = visitEachChild(node, visitor, context);
+                currentSourceFile = undefined;
             }
 
             return node;
@@ -2550,7 +2549,7 @@ namespace ts {
 
             const buildResult = buildStatements();
             return createCall(
-                createIdentifier("__generator"),
+                createHelperName(currentSourceFile.externalHelpersModuleName, "__generator"),
                 /*typeArguments*/ undefined,
                 [setNodeEmitFlags(
                     createFunctionExpression(
diff --git a/tests/baselines/reference/disallowAsyncModifierInES5.errors.txt b/tests/baselines/reference/disallowAsyncModifierInES5.errors.txt
deleted file mode 100644
index eb3cc8218d1..00000000000
--- a/tests/baselines/reference/disallowAsyncModifierInES5.errors.txt
+++ /dev/null
@@ -1,27 +0,0 @@
-error TS2318: Cannot find global type 'Promise'.
-tests/cases/compiler/disallowAsyncModifierInES5.ts(2,1): error TS1311: Async functions are only available when targeting ECMAScript 2015 or higher.
-tests/cases/compiler/disallowAsyncModifierInES5.ts(2,16): error TS1057: An async function or method must have a valid awaitable return type.
-tests/cases/compiler/disallowAsyncModifierInES5.ts(3,11): error TS1057: An async function or method must have a valid awaitable return type.
-tests/cases/compiler/disallowAsyncModifierInES5.ts(3,11): error TS1311: Async functions are only available when targeting ECMAScript 2015 or higher.
-tests/cases/compiler/disallowAsyncModifierInES5.ts(4,11): error TS1311: Async functions are only available when targeting ECMAScript 2015 or higher.
-tests/cases/compiler/disallowAsyncModifierInES5.ts(4,11): error TS1057: An async function or method must have a valid awaitable return type.
-
-
-!!! error TS2318: Cannot find global type 'Promise'.
-==== tests/cases/compiler/disallowAsyncModifierInES5.ts (6 errors) ====
-    
-    async function foo() { return 42; } // ERROR: Async functions are only available in ES6+
-    ~~~~~
-!!! error TS1311: Async functions are only available when targeting ECMAScript 2015 or higher.
-                   ~~~
-!!! error TS1057: An async function or method must have a valid awaitable return type.
-    let bar = async function () { return 42; } // OK, but should be an error
-              ~~~~~
-!!! error TS1057: An async function or method must have a valid awaitable return type.
-              ~~~~~
-!!! error TS1311: Async functions are only available when targeting ECMAScript 2015 or higher.
-    let baz = async () => 42; // OK, but should be an error
-              ~~~~~
-!!! error TS1311: Async functions are only available when targeting ECMAScript 2015 or higher.
-              ~~~~~~~~~~~~~~
-!!! error TS1057: An async function or method must have a valid awaitable return type.
\ No newline at end of file
diff --git a/tests/baselines/reference/disallowAsyncModifierInES5.js b/tests/baselines/reference/disallowAsyncModifierInES5.js
deleted file mode 100644
index e0997f13110..00000000000
--- a/tests/baselines/reference/disallowAsyncModifierInES5.js
+++ /dev/null
@@ -1,23 +0,0 @@
-//// [disallowAsyncModifierInES5.ts]
-
-async function foo() { return 42; } // ERROR: Async functions are only available in ES6+
-let bar = async function () { return 42; } // OK, but should be an error
-let baz = async () => 42; // OK, but should be an error
-
-//// [disallowAsyncModifierInES5.js]
-var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, generator) {
-    return new (P || (P = Promise))(function (resolve, reject) {
-        function fulfilled(value) { try { step(generator.next(value)); } catch (e) { reject(e); } }
-        function rejected(value) { try { step(generator.throw(value)); } catch (e) { reject(e); } }
-        function step(result) { result.done ? resolve(result.value) : new P(function (resolve) { resolve(result.value); }).then(fulfilled, rejected); }
-        step((generator = generator.apply(thisArg, _arguments)).next());
-    });
-};
-var _this = this;
-function foo() {
-    return __awaiter(this, void 0, void 0, function* () { return 42; });
-} // ERROR: Async functions are only available in ES6+
-var bar = function () {
-    return __awaiter(this, void 0, void 0, function* () { return 42; });
-}; // OK, but should be an error
-var baz = function () { return __awaiter(_this, void 0, void 0, function* () { return 42; }); }; // OK, but should be an error
diff --git a/tests/baselines/reference/es5-asyncFunction.js b/tests/baselines/reference/es5-asyncFunction.js
index 28197f55bae..57c9a7c05f6 100644
--- a/tests/baselines/reference/es5-asyncFunction.js
+++ b/tests/baselines/reference/es5-asyncFunction.js
@@ -18,51 +18,40 @@ var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, ge
     });
 };
 var __generator = (this && this.__generator) || function (body) {
-    var _ = { label: 0, sent: function() { if (sent[0] === 1 /*throw*/) throw sent[1]; return sent[1]; }, trys: [], stack: [] }, sent;
+    var _ = { label: 0, sent: function() { if (sent[0] === 1) throw sent[1]; return sent[1]; }, trys: [], stack: [] }, sent, f;
     function step(op) {
-        if (_.flag) throw new TypeError("Generator is already executing.");
-        while (true) {
+        if (f) throw new TypeError("Generator is already executing.");
+        while (1) {
             if (_.done) switch (op[0]) {
-                case 0 /*next*/: return { value: void 0, done: true };
-                case 1 /*throw*/: case 6 /*catch*/: throw op[1];
-                case 2 /*return*/: return { value: op[1], done: true };
+                case 0: return { value: void 0, done: true };
+                case 1: case 6: throw op[1];
+                case 2: return { value: op[1], done: true };
             }
             try {
-                switch (_.flag = true, op[0]) {
-                    case 0 /*next*/: case 1 /*throw*/: sent = op; break;
-                    case 4 /*yield*/: return _.label++, { value: op[1], done: false };
-                    case 7 /*endfinally*/: op = _.stack.pop(), _.trys.pop(); continue;
+                switch (f = 1, op[0]) {
+                    case 0: case 1: sent = op; break;
+                    case 4: return _.label++, { value: op[1], done: false };
+                    case 7: op = _.stack.pop(), _.trys.pop(); continue;
                     default:
                         var r = _.trys.length > 0 && _.trys[_.trys.length - 1];
-                        if (!r && (op[0] === 1 /*throw*/ || op[0] === 6 /*catch*/ || op[0] === 2 /*return*/)) {
-                            _.done = true;
-                            continue;
-                        }
-                        if (op[0] === 3 /*break*/ && (!r || (op[1] > r[0] && op[1] < r[3]))) {
-                            _.label = op[1];
-                        }
-                        else if (op[0] === 6 /*catch*/ && r && _.label < r[1]) {
-                            _.label = r[1], sent = op;
-                        }
-                        else if (r && _.label < r[2]) {
-                            _.label = r[2], _.stack.push(op);
-                        }
-                        else {
-                            if (r[2]) _.stack.pop();
-                            _.trys.pop();
-                            continue;
-                        }
+                        if (!r && (op[0] === 6 || op[0] === 2)) { _.done = 1; continue; }
+                        if (op[0] === 3 && (!r || (op[1] > r[0] && op[1] < r[3]))) { _.label = op[1]; break; }
+                        if (op[0] === 6 && _.label < r[1]) { _.label = r[1], sent = op; break; }
+                        if (r && _.label < r[2]) { _.label = r[2], _.stack.push(op); break; }
+                        if (r[2]) { _.stack.pop(); }
+                        _.trys.pop();
+                        continue;
                 }
                 op = body(_);
             }
-            catch (e) { op = [6 /*catch*/, e]; }
-            finally { _.flag = false, sent = void 0; }
+            catch (e) { op = [6, e]; }
+            finally { f = 0, sent = void 0; }
         }
     }
     return {
-        next: function (v) { return step([0 /*next*/, v]); },
-        "throw": function (v) { return step([1 /*throw*/, v]); },
-        "return": function (v) { return step([2 /*return*/, v]); }
+        next: function (v) { return step([0, v]); },
+        "throw": function (v) { return step([1, v]); },
+        "return": function (v) { return step([2, v]); }
     };
 };
 function empty() {
diff --git a/tests/baselines/reference/es5-importHelpersAsyncFunctions.js b/tests/baselines/reference/es5-importHelpersAsyncFunctions.js
new file mode 100644
index 00000000000..b90e2248c2c
--- /dev/null
+++ b/tests/baselines/reference/es5-importHelpersAsyncFunctions.js
@@ -0,0 +1,83 @@
+//// [tests/cases/compiler/es5-importHelpersAsyncFunctions.ts] ////
+
+//// [external.ts]
+export async function foo() {
+}
+
+//// [script.ts]
+async function foo() {
+}
+
+//// [tslib.d.ts]
+export declare function __extends(d: Function, b: Function): void;
+export declare function __assign(t: any, ...sources: any[]): any;
+export declare function __decorate(decorators: Function[], target: any, key?: string | symbol, desc?: any): any;
+export declare function __param(paramIndex: number, decorator: Function): Function;
+export declare function __metadata(metadataKey: any, metadataValue: any): Function;
+export declare function __awaiter(thisArg: any, _arguments: any, P: Function, generator: Function): any;
+export declare function __generator(body: Function): any;
+
+//// [external.js]
+"use strict";
+var tslib_1 = require("tslib");
+function foo() {
+    return tslib_1.__awaiter(this, void 0, void 0, function () {
+        return tslib_1.__generator(function (_a) {
+            return [2 /*return*/];
+        });
+    });
+}
+exports.foo = foo;
+//// [script.js]
+var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, generator) {
+    return new (P || (P = Promise))(function (resolve, reject) {
+        function fulfilled(value) { try { step(generator.next(value)); } catch (e) { reject(e); } }
+        function rejected(value) { try { step(generator.throw(value)); } catch (e) { reject(e); } }
+        function step(result) { result.done ? resolve(result.value) : new P(function (resolve) { resolve(result.value); }).then(fulfilled, rejected); }
+        step((generator = generator.apply(thisArg, _arguments)).next());
+    });
+};
+var __generator = (this && this.__generator) || function (body) {
+    var _ = { label: 0, sent: function() { if (sent[0] === 1) throw sent[1]; return sent[1]; }, trys: [], stack: [] }, sent, f;
+    function step(op) {
+        if (f) throw new TypeError("Generator is already executing.");
+        while (1) {
+            if (_.done) switch (op[0]) {
+                case 0: return { value: void 0, done: true };
+                case 1: case 6: throw op[1];
+                case 2: return { value: op[1], done: true };
+            }
+            try {
+                switch (f = 1, op[0]) {
+                    case 0: case 1: sent = op; break;
+                    case 4: return _.label++, { value: op[1], done: false };
+                    case 7: op = _.stack.pop(), _.trys.pop(); continue;
+                    default:
+                        var r = _.trys.length > 0 && _.trys[_.trys.length - 1];
+                        if (!r && (op[0] === 6 || op[0] === 2)) { _.done = 1; continue; }
+                        if (op[0] === 3 && (!r || (op[1] > r[0] && op[1] < r[3]))) { _.label = op[1]; break; }
+                        if (op[0] === 6 && _.label < r[1]) { _.label = r[1], sent = op; break; }
+                        if (r && _.label < r[2]) { _.label = r[2], _.stack.push(op); break; }
+                        if (r[2]) { _.stack.pop(); }
+                        _.trys.pop();
+                        continue;
+                }
+                op = body(_);
+            }
+            catch (e) { op = [6, e]; }
+            finally { f = 0, sent = void 0; }
+        }
+    }
+    return {
+        next: function (v) { return step([0, v]); },
+        "throw": function (v) { return step([1, v]); },
+        "return": function (v) { return step([2, v]); }
+    };
+};
+function foo() {
+    return __awaiter(this, void 0, void 0, function () {
+        return __generator(function (_a) {
+            return [2 /*return*/];
+        });
+    });
+}
diff --git a/tests/baselines/reference/es5-importHelpersAsyncFunctions.symbols b/tests/baselines/reference/es5-importHelpersAsyncFunctions.symbols
new file mode 100644
index 00000000000..89660cea722
--- /dev/null
+++ b/tests/baselines/reference/es5-importHelpersAsyncFunctions.symbols
@@ -0,0 +1,58 @@
+=== tests/cases/compiler/external.ts ===
+export async function foo() {
+>foo : Symbol(foo, Decl(external.ts, 0, 0))
+}
+
+=== tests/cases/compiler/script.ts ===
+async function foo() {
+>foo : Symbol(foo, Decl(script.ts, 0, 0))
+}
+
+=== tests/cases/compiler/tslib.d.ts ===
+export declare function __extends(d: Function, b: Function): void;
+>__extends : Symbol(__extends, Decl(tslib.d.ts, --, --))
+>d : Symbol(d, Decl(tslib.d.ts, --, --))
+>Function : Symbol(Function, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>b : Symbol(b, Decl(tslib.d.ts, --, --))
+>Function : Symbol(Function, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
+export declare function __assign(t: any, ...sources: any[]): any;
+>__assign : Symbol(__assign, Decl(tslib.d.ts, --, --))
+>t : Symbol(t, Decl(tslib.d.ts, --, --))
+>sources : Symbol(sources, Decl(tslib.d.ts, --, --))
+
+export declare function __decorate(decorators: Function[], target: any, key?: string | symbol, desc?: any): any;
+>__decorate : Symbol(__decorate, Decl(tslib.d.ts, --, --))
+>decorators : Symbol(decorators, Decl(tslib.d.ts, --, --))
+>Function : Symbol(Function, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>target : Symbol(target, Decl(tslib.d.ts, --, --))
+>key : Symbol(key, Decl(tslib.d.ts, --, --))
+>desc : Symbol(desc, Decl(tslib.d.ts, --, --))
+
+export declare function __param(paramIndex: number, decorator: Function): Function;
+>__param : Symbol(__param, Decl(tslib.d.ts, --, --))
+>paramIndex : Symbol(paramIndex, Decl(tslib.d.ts, --, --))
+>decorator : Symbol(decorator, Decl(tslib.d.ts, --, --))
+>Function : Symbol(Function, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>Function : Symbol(Function, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
+export declare function __metadata(metadataKey: any, metadataValue: any): Function;
+>__metadata : Symbol(__metadata, Decl(tslib.d.ts, --, --))
+>metadataKey : Symbol(metadataKey, Decl(tslib.d.ts, --, --))
+>metadataValue : Symbol(metadataValue, Decl(tslib.d.ts, --, --))
+>Function : Symbol(Function, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
+export declare function __awaiter(thisArg: any, _arguments: any, P: Function, generator: Function): any;
+>__awaiter : Symbol(__awaiter, Decl(tslib.d.ts, --, --))
+>thisArg : Symbol(thisArg, Decl(tslib.d.ts, --, --))
+>_arguments : Symbol(_arguments, Decl(tslib.d.ts, --, --))
+>P : Symbol(P, Decl(tslib.d.ts, --, --))
+>Function : Symbol(Function, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>generator : Symbol(generator, Decl(tslib.d.ts, --, --))
+>Function : Symbol(Function, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
+export declare function __generator(body: Function): any;
+>__generator : Symbol(__generator, Decl(tslib.d.ts, --, --))
+>body : Symbol(body, Decl(tslib.d.ts, --, --))
+>Function : Symbol(Function, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
diff --git a/tests/baselines/reference/es5-importHelpersAsyncFunctions.types b/tests/baselines/reference/es5-importHelpersAsyncFunctions.types
new file mode 100644
index 00000000000..9ceeee92bc8
--- /dev/null
+++ b/tests/baselines/reference/es5-importHelpersAsyncFunctions.types
@@ -0,0 +1,58 @@
+=== tests/cases/compiler/external.ts ===
+export async function foo() {
+>foo : () => Promise<void>
+}
+
+=== tests/cases/compiler/script.ts ===
+async function foo() {
+>foo : () => Promise<void>
+}
+
+=== tests/cases/compiler/tslib.d.ts ===
+export declare function __extends(d: Function, b: Function): void;
+>__extends : (d: Function, b: Function) => void
+>d : Function
+>Function : Function
+>b : Function
+>Function : Function
+
+export declare function __assign(t: any, ...sources: any[]): any;
+>__assign : (t: any, ...sources: any[]) => any
+>t : any
+>sources : any[]
+
+export declare function __decorate(decorators: Function[], target: any, key?: string | symbol, desc?: any): any;
+>__decorate : (decorators: Function[], target: any, key?: string | symbol, desc?: any) => any
+>decorators : Function[]
+>Function : Function
+>target : any
+>key : string | symbol
+>desc : any
+
+export declare function __param(paramIndex: number, decorator: Function): Function;
+>__param : (paramIndex: number, decorator: Function) => Function
+>paramIndex : number
+>decorator : Function
+>Function : Function
+>Function : Function
+
+export declare function __metadata(metadataKey: any, metadataValue: any): Function;
+>__metadata : (metadataKey: any, metadataValue: any) => Function
+>metadataKey : any
+>metadataValue : any
+>Function : Function
+
+export declare function __awaiter(thisArg: any, _arguments: any, P: Function, generator: Function): any;
+>__awaiter : (thisArg: any, _arguments: any, P: Function, generator: Function) => any
+>thisArg : any
+>_arguments : any
+>P : Function
+>Function : Function
+>generator : Function
+>Function : Function
+
+export declare function __generator(body: Function): any;
+>__generator : (body: Function) => any
+>body : Function
+>Function : Function
+
diff --git a/tests/cases/compiler/disallowAsyncModifierInES5.ts b/tests/cases/compiler/disallowAsyncModifierInES5.ts
deleted file mode 100644
index 5417e678dce..00000000000
--- a/tests/cases/compiler/disallowAsyncModifierInES5.ts
+++ /dev/null
@@ -1,5 +0,0 @@
-// @target: es5
-
-async function foo() { return 42; } // ERROR: Async functions are only available in ES6+
-let bar = async function () { return 42; } // OK, but should be an error
-let baz = async () => 42; // OK, but should be an error
\ No newline at end of file
diff --git a/tests/cases/compiler/es5-importHelpersAsyncFunctions.ts b/tests/cases/compiler/es5-importHelpersAsyncFunctions.ts
new file mode 100644
index 00000000000..87acc67a59b
--- /dev/null
+++ b/tests/cases/compiler/es5-importHelpersAsyncFunctions.ts
@@ -0,0 +1,21 @@
+// @lib: es5,es2015.promise
+// @importHelpers: true
+// @target: es5
+// @module: commonjs
+// @moduleResolution: classic
+// @filename: external.ts
+export async function foo() {
+}
+
+// @filename: script.ts
+async function foo() {
+}
+
+// @filename: tslib.d.ts
+export declare function __extends(d: Function, b: Function): void;
+export declare function __assign(t: any, ...sources: any[]): any;
+export declare function __decorate(decorators: Function[], target: any, key?: string | symbol, desc?: any): any;
+export declare function __param(paramIndex: number, decorator: Function): Function;
+export declare function __metadata(metadataKey: any, metadataValue: any): Function;
+export declare function __awaiter(thisArg: any, _arguments: any, P: Function, generator: Function): any;
+export declare function __generator(body: Function): any;
\ No newline at end of file