Support export default for target=ES5/module=ES6.

Fixes #10855

src/compiler/transformers/es6.ts

@@ -541,7 +541,7 @@ namespace ts {
          *
          * @param node A ClassDeclaration node.
          */
-        function visitClassDeclaration(node: ClassDeclaration): Statement {
+        function visitClassDeclaration(node: ClassDeclaration): VisitResult<Statement> {
             // [source]
             //      class C { }
             //
@@ -552,8 +552,17 @@ namespace ts {
             //          return C;
             //      }());
 
+            const modifierFlags = getModifierFlags(node);
+            const isExported = modifierFlags & ModifierFlags.Export;
+            const isDefault = modifierFlags & ModifierFlags.Default;
+
+            // Add an `export` modifier to the statement if needed (for `--target es5 --module es6`)
+            const modifiers = isExported && !isDefault
+                ? filter(node.modifiers, isExportModifier)
+                : undefined;
+
             const statement = createVariableStatement(
-                /*modifiers*/ undefined,
+                modifiers,
                 createVariableDeclarationList([
                     createVariableDeclaration(
                         getDeclarationName(node, /*allowComments*/ true),
@@ -566,9 +575,26 @@ namespace ts {
 
             setOriginalNode(statement, node);
             startOnNewLine(statement);
+
+            // Add an `export default` statement for default exports (for `--target es5 --module es6`)
+            if (isExported && isDefault) {
+                const statements: Statement[] = [statement];
+                statements.push(createExportAssignment(
+                    /*decorators*/ undefined,
+                    /*modifiers*/ undefined,
+                    /*isExportEquals*/ false,
+                    getDeclarationName(node, /*allowComments*/ false)
+                ));
+                return statements;
+            }
+
             return statement;
         }
 
+        function isExportModifier(node: Modifier) {
+            return node.kind === SyntaxKind.ExportKeyword;
+        }
+
         /**
          * Visits a ClassExpression and transforms it into an expression.
          *

tests/baselines/reference/es5andes6module.js

@@ -23,3 +23,4 @@ var A = (function () {
     };
     return A;
 }());
+export default A;

tests/baselines/reference/es6modulekindWithES5Target.js

@@ -27,7 +27,7 @@ var __decorate = (this && this.__decorate) || function (decorators, target, key,
     else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
     return c > 3 && r && Object.defineProperty(target, key, r), r;
 };
-var C = (function () {
+export var C = (function () {
     function C() {
         this.p = 1;
     }

tests/baselines/reference/es6modulekindWithES5Target2.js

@@ -15,4 +15,5 @@ var C = (function () {
     C.prototype.method = function () { };
     return C;
 }());
+export default C;
 C.s = 0;

tests/baselines/reference/externModule.js

@@ -43,7 +43,7 @@ n=XDate.UTC(1964,2,1);
 declare;
 module;
 {
-    var XDate = (function () {
+    export var XDate = (function () {
         function XDate() {
         }
         return XDate;

tests/baselines/reference/moduleElementsInWrongContext.js

@@ -44,6 +44,7 @@
         }
         return C;
     }());
+    export default C;
     function bee() { }
     import I2 = require("foo");
     import * as Foo from "ambient";

tests/baselines/reference/moduleElementsInWrongContext2.js

@@ -44,6 +44,7 @@ function blah() {
         }
         return C;
     }());
+    export default C;
     function bee() { }
     import I2 = require("foo");
     import * as Foo from "ambient";