Introduce a UnionTypeMask, currently limited to 4 types.

tests/compiler/dart2js/simple_inferrer_no_such_method_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 TEST1 = """
@@ skipping 53 matching lines @@
 test5() => (a ? new B() : new D()).foo();
 
 // Can hit A.noSuchMethod, D.noSuchMethod and Object.noSuchMethod.
 test6() => a.bar();
 
 // Can hit A.noSuchMethod.
 test7() => new B().bar();
 test8() => new C().bar();
 test9() => (a ? new B() : new C()).bar();
 
-// Can hit A.noSuchMethod, D.noSuchMethod and Object.noSuchMethod.
+// Can hit A.noSuchMethod and D.noSuchMethod.
 test10() => (a ? new B() : new D()).bar();
 
 // Can hit D.noSuchMethod.
 test11() => new D().bar();
 
 main() {
   test1();
   test2();
   test3();
   test4();
   test5();
   test6();
   test7();
   test8();
   test9();
   test10();
   test11();
 }
 """;
 
 main() {
   Uri uri = new Uri.fromComponents(scheme: 'source');
 
   var compiler = compilerFor(TEST1, 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);
   }
 
   checkReturn('test1', typesInferrer.intType);
   checkReturn('test2', typesInferrer.dynamicType);
   checkReturn('test3', typesInferrer.intType);
   checkReturn('test4', typesInferrer.mapType);
-  checkReturn('test5', typesInferrer.dynamicType);
-  checkReturn('test6', typesInferrer.dynamicType);
+  checkReturn('test5', typesInferrer.dynamicType.nonNullable());
+  checkReturn('test6', typesInferrer.dynamicType.nonNullable());
 
   compiler = compilerFor(TEST2, uri);
   compiler.runCompiler(uri);
   typesInferrer = compiler.typesTask.typesInferrer;
 
   checkReturn('test1', typesInferrer.dynamicType);
   checkReturn('test2', typesInferrer.mapType);
   checkReturn('test3', typesInferrer.mapType);
   checkReturn('test4', typesInferrer.mapType);
   checkReturn('test5', typesInferrer.mapType);
 
   // TODO(ngeoffray): The reason for nullablity is because the
   // inferrer thinks Object.noSuchMethod return null. Once we track
   // aborting control flow in the analysis, we won't get the nullable
   // anymore.
   checkReturn('test6', typesInferrer.numType.nullable());
   checkReturn('test7', typesInferrer.intType);
   checkReturn('test8', typesInferrer.intType);
   checkReturn('test9', typesInferrer.intType);
-  checkReturn('test10', typesInferrer.numType.nullable());
+  checkReturn('test10', typesInferrer.numType);
   checkReturn('test11', typesInferrer.doubleType);
 }