Make identical(a,b) a compile-time constant in dart2js.

tests/compiler/dart2js/mock_compiler.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.
 
 #library('mock_compiler');
 
 #import("dart:uri");
 
 #import("../../../lib/compiler/implementation/elements/elements.dart");
 #import("../../../lib/compiler/implementation/leg.dart");
@@ skipping 46 matching lines @@
   interface int extends num {}
   interface double extends num {}
   class bool {}
   class String {}
   class Object {}
   interface num {}
   class Function {}
   class List {}
   class Closure {}
   class Null {}
-  class Dynamic_ {}''';
+  class Dynamic_ {}
+  bool identical(Object a, Object b) {}''';
 
 class MockCompiler extends Compiler {
   List<WarningMessage> warnings;
   List<WarningMessage> errors;
   final Map<String, SourceFile> sourceFiles;
   Node parsedTree;
 
   MockCompiler([String coreSource = DEFAULT_CORELIB,
                 String helperSource = DEFAULT_HELPERLIB,
                 String interceptorsSource = DEFAULT_INTERCEPTORSLIB,
@@ skipping 135 matching lines @@
     });
   }
 }
 
 LibraryElement mockLibrary(Compiler compiler, String source) {
   Uri uri = new Uri.fromComponents(scheme: "source");
   var library = new LibraryElement(new Script(uri, new MockFile(source)));
   importLibrary(library, compiler.coreLibrary, compiler);
   return library;
 }