Handle @proxy classes in type checker.

tests/utils/dummy_compiler_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.
 // VMOptions=
 // VMOptions=--print-object-histogram
 
 // Smoke test of the dart2js compiler API.
 library dummy_compiler;
 
 import 'dart:async';
@@ skipping 26 matching lines @@
 class Closure {}
 class BoundClosure {}
 class Dynamic_ {}
 class Null {}
 class StackTrace {}
 class LinkedHashMap {}
 identical(a, b) => true;
 getRuntimeTypeInfo(o) {}
 setRuntimeTypeInfo(o, i) {}
 eqNull(a) {}
-eqNullB(a) {}""";
+eqNullB(a) {}
+const proxy = 0;""";
     } else if (uri.path.endsWith('_patch.dart')) {
       source = '';
     } else if (uri.path.endsWith('interceptors.dart')) {
       source = """
 class Interceptor {
   operator==(other) {}
   get hashCode => throw 'Interceptor.hashCode not implemented.';
 }
 class JSIndexable {
   get length {}
@@ skipping 58 matching lines @@
               new Uri(scheme: 'package', path: '/'),
               provider, handler);
   result.then((String code) {
     if (code == null) {
       throw 'Compilation failed';
     }
   }, onError: (e) {
       throw 'Compilation failed';
   }).then(asyncSuccess);
 }