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

tests/compiler/dart2js/union_type_test.dart

 // Copyright (c) 2013, 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 "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
 import "package:compiler/src/types/types.dart";
-
-import "compiler_helper.dart";
+import "package:compiler/src/world.dart";
+import 'type_test_helper.dart';
 
 main() {
-  MockCompiler compiler = new MockCompiler.internal(analyzeOnly: true);
-  asyncTest(() => compiler.run(null, """
+
+  asyncTest(() async {
+    TypeEnvironment env = await TypeEnvironment.create(r"""
+      class A {}
+      class B {}
+      """,
+      mainSource: r"""
       main() {
-        print(2); print("Hello");
+        new A();
+        new B();
       }
-    """).then((_) {
-    FlatTypeMask mask1 =
-        new FlatTypeMask.exact(compiler.coreClasses.intClass);
-    FlatTypeMask mask2 =
-        new FlatTypeMask.exact(compiler.coreClasses.stringClass);
-    UnionTypeMask union1 = mask1.nonNullable().union(mask2, compiler.world);
-    UnionTypeMask union2 = mask2.nonNullable().union(mask1, compiler.world);
+      """,
+      useMockCompiler: false);
+    World world = env.compiler.world;
+    world.populate();
+    FlatTypeMask mask1 = new FlatTypeMask.exact(env.getElement('A'));
+    FlatTypeMask mask2 = new FlatTypeMask.exact(env.getElement('B'));
+    UnionTypeMask union1 = mask1.nonNullable().union(mask2, world);
+    UnionTypeMask union2 = mask2.nonNullable().union(mask1, world);
     Expect.equals(union1, union2);
-  }));
+  });
 }