Add renames in output when using the mirror helper library.

tests/compiler/dart2js/dart_backend_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import 'parser_helper.dart';
 import 'mock_compiler.dart';
 import '../../../sdk/lib/_internal/compiler/compiler.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/dart2jslib.dart' as leg;
@@ skipping 436 matching lines @@
   new PlaceholderCollector(compiler, new Set<String>(), new DynoMap(compiler))
       ..collect(element);
 
 testLocalFunctionPlaceholder() {
   var src = '''
 main() {
   function localfoo() {}
   localfoo();
 }
 ''';
-  MockCompiler compiler = new MockCompiler();
+  MockCompiler compiler = new MockCompiler(emitJavaScript: false);
+  assert(compiler.backend is DartBackend);
   compiler.parseScript(src);
   FunctionElement mainElement = compiler.mainApp.find(leg.Compiler.MAIN);
   compiler.processQueue(compiler.enqueuer.resolution, mainElement);
   PlaceholderCollector collector = collectPlaceholders(compiler, mainElement);
   FunctionExpression mainNode = mainElement.parseNode(compiler);
   FunctionExpression fooNode = mainNode.body.statements.nodes.head.function;
   LocalPlaceholder fooPlaceholder =
       collector.functionScopes[mainElement].localPlaceholders.first;
   Expect.isTrue(fooPlaceholder.nodes.contains(fooNode.name));
 }
@@ skipping 257 matching lines @@
   testStaticAccessIoLib();
   testLocalFunctionPlaceholder();
   testMinification();
   testClosureLocalsMinified();
   testParametersMinified();
   testDeclarationTypePlaceholders();
   testPlatformLibraryMemberNamesAreFixed();
   testConflictsWithCoreLib();
   testUnresolvedNamedConstructor();
 }