Disallow JS/non-JS merge without crashing

Note that the error location is misleading because it's reported inside
the merge step for the js initializer.

src/compiler/utilities.ts

@@ -1489,7 +1489,7 @@ namespace ts {
         }
         const declaration = symbol.valueDeclaration;
         const e = getDeclaredJavascriptInitializer(declaration) || getAssignedJavascriptInitializer(declaration);
-        return e ? e.symbol : symbol;
+        return e && e.symbol ? e.symbol : symbol;
     }
 
     /** Get the declaration initializer, when the initializer is container-like (See getJavascriptInitializer) */

tests/baselines/reference/jsContainerMergeTsDeclaration.errors.txt

@@ -0,0 +1,22 @@
+error TS5055: Cannot write file 'tests/cases/conformance/salsa/a.js' because it would overwrite input file.
+  Adding a tsconfig.json file will help organize projects that contain both TypeScript and JavaScript files. Learn more at https://aka.ms/tsconfig.
+tests/cases/conformance/salsa/a.js(1,23): error TS2300: Duplicate identifier 'x'.
+tests/cases/conformance/salsa/b.ts(1,5): error TS2300: Duplicate identifier 'x'.
+
+
+!!! error TS5055: Cannot write file 'tests/cases/conformance/salsa/a.js' because it would overwrite input file.
+!!! error TS5055:   Adding a tsconfig.json file will help organize projects that contain both TypeScript and JavaScript files. Learn more at https://aka.ms/tsconfig.
+==== tests/cases/conformance/salsa/a.js (1 errors) ====
+    var /*1*/x = function foo() {
+                          ~~~
+!!! error TS2300: Duplicate identifier 'x'.
+    }
+    x.a = function bar() {
+    }
+==== tests/cases/conformance/salsa/b.ts (1 errors) ====
+    var x = function () {
+        ~
+!!! error TS2300: Duplicate identifier 'x'.
+        return 1;
+    }();
+    

tests/baselines/reference/jsContainerMergeTsDeclaration.js

@@ -0,0 +1,17 @@
+//// [tests/cases/conformance/salsa/jsContainerMergeTsDeclaration.ts] ////
+
+//// [a.js]
+var /*1*/x = function foo() {
+}
+x.a = function bar() {
+}
+//// [b.ts]
+var x = function () {
+    return 1;
+}();
+
+
+//// [b.js]
+var x = function () {
+    return 1;
+}();

tests/baselines/reference/jsContainerMergeTsDeclaration.symbols

@@ -0,0 +1,18 @@
+=== tests/cases/conformance/salsa/a.js ===
+var /*1*/x = function foo() {
+>x : Symbol(x, Decl(a.js, 0, 3), Decl(a.js, 1, 1), Decl(b.ts, 0, 3))
+>foo : Symbol(foo, Decl(a.js, 0, 12))
+}
+x.a = function bar() {
+>x.a : Symbol(foo.a, Decl(a.js, 1, 1))
+>x : Symbol(x, Decl(a.js, 0, 3), Decl(a.js, 1, 1), Decl(b.ts, 0, 3))
+>a : Symbol(foo.a, Decl(a.js, 1, 1))
+>bar : Symbol(bar, Decl(a.js, 2, 5))
+}
+=== tests/cases/conformance/salsa/b.ts ===
+var x = function () {
+>x : Symbol(x, Decl(a.js, 0, 3), Decl(a.js, 1, 1), Decl(b.ts, 0, 3))
+
+    return 1;
+}();
+

tests/baselines/reference/jsContainerMergeTsDeclaration.types

@@ -0,0 +1,25 @@
+=== tests/cases/conformance/salsa/a.js ===
+var /*1*/x = function foo() {
+>x : { (): void; a: () => void; }
+>function foo() {} : { (): void; a: () => void; }
+>foo : { (): void; a: () => void; }
+}
+x.a = function bar() {
+>x.a = function bar() {} : () => void
+>x.a : () => void
+>x : { (): void; a: () => void; }
+>a : () => void
+>function bar() {} : () => void
+>bar : () => void
+}
+=== tests/cases/conformance/salsa/b.ts ===
+var x = function () {
+>x : { (): void; a: () => void; }
+>function () {    return 1;}() : number
+>function () {    return 1;} : () => number
+
+    return 1;
+>1 : 1
+
+}();
+

tests/cases/conformance/salsa/jsContainerMergeTsDeclaration.ts

@@ -0,0 +1,11 @@
+// @allowJs: true
+// @checkJs: true
+// @Filename: a.js
+var /*1*/x = function foo() {
+}
+x.a = function bar() {
+}
+// @Filename: b.ts
+var x = function () {
+    return 1;
+}();