Introduce a UnionTypeMask, currently limited to 4 types.

tests/utils/dummy_compiler_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.
 
 // Smoke test of the dart2js compiler API.
 library dummy_compiler;
 
 import 'dart:async';
 import 'dart:uri';
 
@@ skipping 34 matching lines @@
                     operator==(other) {}
                   }
                   class JSIndexable {
                     get length;
                   }
                   class JSMutableIndexable {}
                   class JSArray {
                     var removeLast;
                     var add;
                   }
-                  class JSFixedArray {}
-                  class JSExtendableArray {}
+                  class JSMutableArray {}
+                  class JSFixedArray extends JSMutableArray {}
+                  class JSExtendableArray extends JSMutableArray {}
                   class JSString {
                     var split;
                     var concat;
                     var toString;
                   }
                   class JSFunction {}
                   class JSInt {}
                   class JSDouble {}
                   class JSNumber {}
                   class JSNull {}
@@ skipping 30 matching lines @@
               new Uri.fromComponents(scheme: 'package', path: '/'),
               provider, handler);
   result.then((String code) {
     if (code == null) {
       throw 'Compilation failed';
     }
   }, onError: (e) {
       throw 'Compilation failed';
   });
 }