Introduce a UnionTypeMask, currently limited to 4 types.

tests/compiler/dart2js/simple_inferrer_and_or_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:expect/expect.dart';
 import 'compiler_helper.dart';
 
 const String TEST = """
 class X {}
 returnDyn1() {
@@ skipping 44 matching lines @@
 
 
 void main() {
   Uri uri = new Uri.fromComponents(scheme: 'source');
   var compiler = compilerFor(TEST, uri);
   compiler.runCompiler(uri);
   var typesInferrer = compiler.typesTask.typesInferrer;
 
   checkReturn(String name, type) {
     var element = findElement(compiler, name);
-    Expect.equals(type, typesInferrer.internal.returnTypeOf[element]);
+    Expect.equals(type,
+        typesInferrer.internal.returnTypeOf[element].simplify(compiler));
   }
 
   var subclassOfInterceptor =
       findTypeMask(compiler, 'Interceptor', 'nonNullSubclass');
 
   checkReturn('returnDyn1', subclassOfInterceptor);
   checkReturn('returnDyn2', subclassOfInterceptor);
   checkReturn('returnDyn3', subclassOfInterceptor);
-  checkReturn('returnDyn4', typesInferrer.dynamicType);
-  checkReturn('returnDyn5', typesInferrer.dynamicType);
-  checkReturn('returnDyn6', typesInferrer.dynamicType);
+  checkReturn('returnDyn4', typesInferrer.dynamicType.nonNullable());
+  checkReturn('returnDyn5', typesInferrer.dynamicType.nonNullable());
+  checkReturn('returnDyn6', typesInferrer.dynamicType.nonNullable());
 }