Handle @proxy classes in type checker.

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 "package:async_helper/async_helper.dart";
 import 'parser_helper.dart';
 import 'mock_compiler.dart';
 import '../../../sdk/lib/_internal/compiler/compiler.dart';
@@ skipping 21 matching lines @@
 class Null {}
 class TypeError {}
 class Type {}
 class StackTrace {}
 class LinkedHashMap {}
 class Math {
   static double parseDouble(String s) => 1.0;
 }
 print(x) {}
 identical(a, b) => true;
+const proxy = 0;
 ''';
 
 const ioLib = r'''
 library io;
 class Platform {
   static int operatingSystem;
 }
 ''';
 
 const htmlLib = r'''
@@ skipping 677 matching lines @@
   testStaticAccessIoLib();
   testLocalFunctionPlaceholder();
   testMinification();
   testClosureLocalsMinified();
   testParametersMinified();
   testDeclarationTypePlaceholders();
   testPlatformLibraryMemberNamesAreFixed();
   testConflictsWithCoreLib();
   testUnresolvedNamedConstructor();
 }