Normalize type masks to use the least upper instantiated subclass/type.

tests/compiler/dart2js/patch_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 'dart:async';
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/diagnostics/messages.dart' show
     MessageKind;
@@ skipping 896 matching lines @@
       }
       """,
       main: """
       main () {
         new A(); // ensure A and B are instantiated
         new B();
       }
       """,
       runCompiler: true, analyzeOnly: true);
   World world = compiler.world;
+  world.populate();
 
   ClassElement cls = ensure(compiler, "A", compiler.coreLibrary.find,
                             expectIsPatched: true);
   cls.ensureResolved(compiler.resolution);
 
   ensure(compiler, "method", cls.patch.lookupLocalMember,
          checkHasBody: true, expectIsRegular: true);
 
   ensure(compiler, "clear", cls.lookupLocalMember,
          checkHasBody: true, expectIsPatched: true);
@@ skipping 152 matching lines @@
     await testPatchNonFunction();
 
     await testPatchAndSelector();
 
     await testEffectiveTarget();
 
     await testAnalyzeAllInjectedMembers();
     await testTypecheckPatchedMembers();
   });
 }