Infer types of final fields.

tests/language/field_override2_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.
+
+// Test that we are accessing the right field in a method of a super
+// class, when that field is overridden.
+
+class A {
+  final a = [42];
+  foo() => a[0];
+}
+
+class B extends A {
+  final a = new Map();
+}
+
+main() {
+  Expect.equals(null, new B().foo());
+  Expect.equals(42, new A().foo());
+}