Introduce a UnionTypeMask, currently limited to 4 types.

tests/compiler/dart2js/simple_inferrer_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
     '../../../sdk/lib/_internal/compiler/implementation/types/types.dart'
     show TypeMask;
 
 import 'compiler_helper.dart';
@@ skipping 425 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], name);
+    Expect.equals(
+        type,
+        typesInferrer.internal.returnTypeOf[element].simplify(compiler),
+        name);
   }
   var interceptorType =
       findTypeMask(compiler, 'Interceptor', 'nonNullSubclass');
 
   checkReturn('returnNum1', typesInferrer.numType);
   checkReturn('returnNum2', typesInferrer.numType);
   checkReturn('returnInt1', typesInferrer.intType);
   checkReturn('returnInt2', typesInferrer.intType);
   checkReturn('returnDouble', typesInferrer.doubleType);
   checkReturn('returnGiveUp', interceptorType);
@@ skipping 27 matching lines @@
   checkReturn('testIsCheck17', intType);
   checkReturn('testIsCheck18', typesInferrer.dynamicType);
   checkReturn('testIsCheck19', typesInferrer.dynamicType);
   checkReturn('returnAsString',
       new TypeMask.subtype(compiler.stringClass.computeType(compiler)));
   checkReturn('returnIntAsNum', typesInferrer.intType);
   checkReturn('returnAsTypedef', typesInferrer.functionType.nullable());
   checkReturn('returnTopLevelGetter', typesInferrer.intType);
   checkReturn('testDeadCode', typesInferrer.intType);
   checkReturn('testLabeledIf', typesInferrer.intType.nullable());
-  checkReturn('testSwitch1',
-    typesInferrer.intType.union(typesInferrer.doubleType, compiler).nullable());
+  checkReturn('testSwitch1', typesInferrer.intType
+      .union(typesInferrer.doubleType, compiler).nullable().simplify(compiler));
   checkReturn('testSwitch2', typesInferrer.intType);
   checkReturn('testSwitch3', interceptorType.nullable());
   checkReturn('testContinue1', interceptorType.nullable());
   checkReturn('testBreak1', interceptorType.nullable());
   checkReturn('testContinue2', interceptorType.nullable());
   checkReturn('testBreak2', typesInferrer.intType.nullable());
 
   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));
   }
 
   checkReturnInClass('A', 'returnInt1', typesInferrer.intType);
   checkReturnInClass('A', 'returnInt2', typesInferrer.intType);
   checkReturnInClass('A', 'returnInt3', typesInferrer.intType);
   checkReturnInClass('A', 'returnInt4', typesInferrer.intType);
   checkReturnInClass('A', 'returnInt5', typesInferrer.intType);
   checkReturnInClass('A', 'returnInt6', typesInferrer.intType);
   checkReturnInClass('A', '==', interceptorType);
 
   checkReturnInClass('B', 'returnInt1', typesInferrer.intType);
   checkReturnInClass('B', 'returnInt2', typesInferrer.intType);
   checkReturnInClass('B', 'returnInt3', typesInferrer.intType);
   checkReturnInClass('B', 'returnInt4', typesInferrer.intType);
   checkReturnInClass('B', 'returnInt5', typesInferrer.intType);
   checkReturnInClass('B', 'returnInt6', typesInferrer.intType);
   checkReturnInClass('B', 'returnInt7', typesInferrer.intType);
   checkReturnInClass('B', 'returnInt8', typesInferrer.intType);
   checkReturnInClass('B', 'returnInt9', typesInferrer.intType);
 
   checkFactoryConstructor(String className) {
     var cls = findElement(compiler, className);
     var element = cls.localLookup(buildSourceString(className));
     Expect.equals(new TypeMask.nonNullExact(cls.rawType),
                   typesInferrer.internal.returnTypeOf[element]);
   }
   checkFactoryConstructor('A');
 }