Handle @proxy classes in type checker.

tests/utils/recursive_import_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.
 
 // Test of "recursive" imports using the dart2js compiler API.
 
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 import 'dart:async';
 import '../../sdk/lib/_internal/compiler/compiler.dart';
 
 const CORE_LIB = """
 library core;
 class Object {
-  Object();
+  const Object();
   operator==(other) {}
 }
 class bool {}
 class num {}
 class int {}
 class double{}
 class String{}
 class Function{}
 class List {}
 class Map {}
 class BoundClosure {}
 class Closure {}
 class Dynamic_ {}
 class Type {}
 class Null {}
 class StackTrace {}
 class LinkedHashMap {}
 getRuntimeTypeInfo(o) {}
 setRuntimeTypeInfo(o, i) {}
 eqNull(a) {}
 eqNullB(a) {}
 class JSInvocationMirror {}  // Should be in helper.
+class _Proxy { const _Proxy(); }
+const proxy = const _Proxy();
 """;
 
 const INTERCEPTORS_LIB = """
 library interceptors;
 class JSIndexable {
   get length;
 }
 class JSMutableIndexable {}
 class JSArray {
   JSArray() {}
@@ skipping 77 matching lines @@
     Expect.isNull(code);
     Expect.isTrue(10 < count);
     // Two warnings for each time RECURSIVE_MAIN is read, except the
     // first time.
     Expect.equals(2 * (count - 1), warningCount);
     Expect.equals(1, errorCount);
   }, onError: (e) {
       throw 'Compilation failed';
   }).then(asyncSuccess);
 }