Only emit "use strict" when users ask for or the file is external module

Author: Yui T

src/compiler/transformers/module/module.ts

@@ -89,7 +89,8 @@ namespace ts {
             startLexicalEnvironment();
 
             const statements: Statement[] = [];
-            const statementOffset = addPrologueDirectives(statements, node.statements, /*ensureUseStrict*/ !compilerOptions.noImplicitUseStrict, sourceElementVisitor);
+            const ensureUseStrict = compilerOptions.alwaysStrict || (!compilerOptions.noImplicitUseStrict && isExternalModule(currentSourceFile));
+            const statementOffset = addPrologueDirectives(statements, node.statements, ensureUseStrict, sourceElementVisitor);
 
             if (shouldEmitUnderscoreUnderscoreESModule()) {
                 append(statements, createUnderscoreUnderscoreESModule());

src/compiler/transformers/module/system.ts

@@ -225,7 +225,8 @@ namespace ts {
             startLexicalEnvironment();
 
             // Add any prologue directives.
-            const statementOffset = addPrologueDirectives(statements, node.statements, /*ensureUseStrict*/ !compilerOptions.noImplicitUseStrict, sourceElementVisitor);
+            const ensureUseStrict = compilerOptions.alwaysStrict || (!compilerOptions.noImplicitUseStrict && isExternalModule(currentSourceFile));
+            const statementOffset = addPrologueDirectives(statements, node.statements, ensureUseStrict, sourceElementVisitor);
 
             // var __moduleName = context_1 && context_1.id;
             statements.push(

tests/baselines/reference/importHelpersInIsolatedModules.js

@@ -68,7 +68,6 @@ C = tslib_1.__decorate([
     dec
 ], C);
 //// [script.js]
-"use strict";
 var tslib_1 = require("tslib");
 var A = (function () {
     function A() {

tests/baselines/reference/isolatedModulesPlainFile-CommonJS.js

@@ -5,5 +5,4 @@ run(1);
 
 
 //// [isolatedModulesPlainFile-CommonJS.js]
-"use strict";
 run(1);

tests/baselines/reference/isolatedModulesPlainFile-System.js

@@ -6,7 +6,6 @@ run(1);
 
 //// [isolatedModulesPlainFile-System.js]
 System.register([], function (exports_1, context_1) {
-    "use strict";
     var __moduleName = context_1 && context_1.id;
     return {
         setters: [],