Fix receiver refining optimization in the presence of null: any Object method can be called on the …

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:collection';
 
 import '../../../sdk/lib/_internal/compiler/compiler.dart' as api;
@@ skipping 136 matching lines @@
     remainder(other) => (this is JSInt) ? 42 : 42.0;
     truncate() => 42;
   }
   class JSInt extends JSNumber implements int {
   }
   class JSDouble extends JSNumber implements double {
   }
   class JSNull extends Interceptor {
     bool operator==(other) => identical(null, other);
     get hashCode => throw "JSNull.hashCode not implemented.";
+    String toString() => 'Null';
+    Type get runtimeType => null;
   }
   class JSBool extends Interceptor implements bool {
   }
   class JSFunction extends Interceptor implements Function {
   }
   class ObjectInterceptor {
   }
   getInterceptor(x) {}
   getNativeInterceptor(x) {}
   var dispatchPropertyName;
@@ skipping 289 matching lines @@
   }
 }
 
 class MockDeferredLoadTask extends DeferredLoadTask {
   MockDeferredLoadTask(Compiler compiler) : super(compiler);
 
   void registerMainApp(LibraryElement mainApp) {
     // Do nothing.
   }
 }