Handle @proxy classes in type checker.

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 "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:collection';
 
@@ skipping 189 matching lines @@
   class List<E> {
     List([length]);
     List.filled(length, element);
   }
   abstract class Map<K,V> {}
   class DateTime {
     DateTime(year);
     DateTime.utc(year);
   }
   abstract class Pattern {}
-  bool identical(Object a, Object b) { return true; }''';
+  bool identical(Object a, Object b) { return true; }
+  const proxy = 0;''';
 
 const String DEFAULT_ISOLATE_HELPERLIB = r'''
   var startRootIsolate;
   var _currentIsolate;
   var _callInIsolate;
   class _WorkerBase {}''';
 
 class MockCompiler extends Compiler {
   api.DiagnosticHandler diagnosticHandler;
   List<WarningMessage> warnings;
@@ skipping 327 matching lines @@
     } else {
       sourceFile = compiler.sourceFiles[uri.toString()];
     }
     if (sourceFile != null && begin != null && end != null) {
       print(sourceFile.getLocationMessage(message, begin, end, true, (x) => x));
     } else {
       print(message);
     }
   };
 }