Make mock_compiler initialize the identical function, and use the identical function element direct…

tests/compiler/dart2js/size_test.dart

 // Copyright (c) 2013, 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 "compiler_helper.dart";
 
 const String TEST = "main() => [];";
 
 const String DEFAULT_CORELIB_WITH_LIST = r'''
   class Object {}
   class bool {}
   abstract class List {}
   class num {}
   class int {}
   class double {}
   class String {}
   class Function {}
   class Type {}
   class Map {}
+  identical(a, b) => true;
 ''';
 
 main() {
   String generated = compileAll(TEST, coreSource: DEFAULT_CORELIB_WITH_LIST);
   MockCompiler compiler = new MockCompiler();
   var backend = compiler.backend;
 
   // Make sure no class is emitted.
   Expect.isFalse(generated.contains(backend.emitter.finishClassesName));
 }