Add testcase - generating declaration file results in more errors in ts file

src/harness/fourslash.ts

@@ -535,6 +535,16 @@ namespace FourSlash {
             }
         }
 
+        public verifyGetEmitOutputContentsForCurrentFile(expected: { fileName: string; content: string; }[]): void {
+            let emit = this.languageService.getEmitOutput(this.activeFile.fileName);
+            this.taoInvalidReason = "verifyGetEmitOutputContentsForCurrentFile impossible";
+            assert.equal(emit.outputFiles.length, expected.length, "Number of emit output files");
+            for (let i = 0; i < emit.outputFiles.length; i++) {
+                assert.equal(emit.outputFiles[i].name, expected[i].fileName, "FileName");
+                assert.equal(emit.outputFiles[i].text, expected[i].content, "Content");
+            }
+        }
+
         public verifyMemberListContains(symbol: string, text?: string, documentation?: string, kind?: string) {
             this.scenarioActions.push("<ShowCompletionList />");
             this.scenarioActions.push(`<VerifyCompletionContainsItem ItemName="${symbol}"/>`);

tests/cases/fourslash/fourslash.ts

@@ -314,6 +314,10 @@ module FourSlashInterface {
             FourSlash.currentTestState.verifyGetEmitOutputForCurrentFile(expected);
         }
 
+        public verifyGetEmitOutputContentsForCurrentFile(expected: { fileName: string; content: string; }[]): void {
+            FourSlash.currentTestState.verifyGetEmitOutputContentsForCurrentFile(expected);
+        }
+
         public currentParameterHelpArgumentNameIs(name: string) {
             FourSlash.currentTestState.verifyCurrentParameterHelpName(name);
         }

tests/cases/fourslash/jsFileCompilationDuplicateFunctionImplementation.ts

@@ -0,0 +1,20 @@
+/// <reference path="fourslash.ts" />
+
+// @declaration: true
+// @out: out.js
+// @jsExtensions: js
+// @Filename: b.js
+// @emitThisFile: true
+////function foo() { return 10; }/*1*/
+
+// @Filename: a.ts
+// @emitThisFile: true
+////function foo() { return 30; }/*2*/
+
+goTo.marker("2");
+verify.getSemanticDiagnostics("[]");
+verify.verifyGetEmitOutputContentsForCurrentFile([
+    { fileName: "out.js", content: "function foo() { return 10; }\r\nfunction foo() { return 30; }\r\n" },
+    { fileName: "out.d.ts", content: "" }]);
+goTo.marker("2");
+verify.getSemanticDiagnostics('[\n  {\n    "message": "Duplicate function implementation.",\n    "start": 9,\n    "length": 3,\n    "category": "error",\n    "code": 2393\n  }\n]');