Introduce a UnionTypeMask, currently limited to 4 types.

tests/compiler/dart2js/simple_inferrer_postfix_prefix_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';
 import 'parser_helper.dart';
 
 const String TEST = """
 
@@ skipping 51 matching lines @@
 
 void main() {
   Uri uri = new Uri.fromComponents(scheme: 'source');
   var compiler = compilerFor(TEST, uri);
   compiler.runCompiler(uri);
   var typesInferrer = compiler.typesTask.typesInferrer;
 
   checkReturnInClass(String className, String methodName, type) {
     var cls = findElement(compiler, className);
     var element = cls.lookupLocalMember(buildSourceString(methodName));
-    Expect.equals(type, typesInferrer.internal.returnTypeOf[element]);
+    Expect.equals(type,
+        typesInferrer.internal.returnTypeOf[element].simplify(compiler));
   }
 
   var subclassOfInterceptor =
       findTypeMask(compiler, 'Interceptor', 'nonNullSubclass');
 
   checkReturnInClass('A', 'returnNum1', typesInferrer.numType);
   checkReturnInClass('A', 'returnNum2', typesInferrer.numType);
   checkReturnInClass('A', 'returnNum3', typesInferrer.numType);
   checkReturnInClass('A', 'returnNum4', typesInferrer.numType);
   checkReturnInClass('A', 'returnNum5', typesInferrer.numType);
   checkReturnInClass('A', 'returnNum6', typesInferrer.numType);
   checkReturnInClass('A', 'returnDynamic1', subclassOfInterceptor);
   checkReturnInClass('A', 'returnDynamic2', subclassOfInterceptor);
   checkReturnInClass('A', 'returnDynamic3', typesInferrer.dynamicType);
 
   checkReturnInClass('B', 'returnString1', typesInferrer.stringType);
   checkReturnInClass('B', 'returnString2', typesInferrer.stringType);
   checkReturnInClass('B', 'returnDynamic1', typesInferrer.dynamicType);
   checkReturnInClass('B', 'returnDynamic2', typesInferrer.dynamicType);
   checkReturnInClass('B', 'returnDynamic3', typesInferrer.dynamicType);
   checkReturnInClass('B', 'returnDynamic4', typesInferrer.dynamicType);
 }