Handle @proxy classes in type checker.

tests/compiler/dart2js/value_range_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 'compiler_helper.dart';
 
 const int REMOVED = 0;
 const int ABOVE_ZERO = 1;
 const int BELOW_LENGTH = 2;
@@ skipping 261 matching lines @@
   class Type {}
   class Function {}
   class List {
     List([int length]);
   }
   abstract class Map {}
   class Closure {}
   class Null {}
   class Dynamic_ {}
   class StackTrace {}
-  bool identical(Object a, Object b) {}''';
+  bool identical(Object a, Object b) {}
+  const proxy = 0;''';
 
 const String INTERCEPTORSLIB_WITH_MEMBERS = r'''
   class Interceptor {
     toString() {}
     bool operator==(other) => identical(this, other);
     noSuchMethod(im) { throw im; }
   }
   abstract class JSIndexable {
     get length;
   }
@@ skipping 89 matching lines @@
       break;
   }
 }
 
 
 main() {
   for (int i = 0; i < TESTS.length;  i += 2) {
     expect(TESTS[i], TESTS[i + 1]);
   }
 }