Start work on a non-complete type inferrer. Currently only analyzes return types.

tests/language/return_this_type_test.dart

Index: tests/language/return_this_type_test.dart
===================================================================
--- tests/language/return_this_type_test.dart	(revision 0)
+++ tests/language/return_this_type_test.dart	(revision 0)
@@ -0,0 +1,16 @@
+// 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.
+
+// Make sure the engine does not infer the wrong type for [:A.foo:].
+
+class A {
+  foo() => this;
+}
+
+class B extends A {
+}
+
+main() {
+  Expect.isTrue(new B().foo() is B);
+}