Fix type masks for js-interop types.

tests/compiler/dart2js/jsinterop/abstract_test.dart

+// Copyright (c) 2016, 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 jsinterop.abstract_test;
+
+import 'package:expect/expect.dart';
+import 'package:async_helper/async_helper.dart';
+import 'package:compiler/src/common.dart';
+import '../memory_compiler.dart';
+
+void main() {
+  asyncTest(() async {
+    DiagnosticCollector collector = new DiagnosticCollector();
+    await runCompiler(
+        diagnosticHandler: collector,
+        memorySourceFiles: const {'main.dart': '''
+import 'package:js/js.dart';
+
+@JS()
+class A {
+  get foo;
+}
+
+main() => new A();
+'''});
+    Expect.equals(0, collector.errors.length,
+        'Unexpected error count.');
+    Expect.equals(1, collector.warnings.length,
+        'Unexpected warning count.');
+    Expect.equals(MessageKind.ABSTRACT_GETTER,
+        collector.warnings.first.messageKind,
+        'Unexpected warning.');
+  });
+}