Allow access to super fields inside a closure.

tests/language/closure_with_super_field_test.dart

+// Copyright (c) 2012, 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.
+
+class A {
+  int a;
+  A(): a = 42;
+}
+
+class B extends A {
+  int get a => 54;
+  returnSuper() => super.a;
+  returnSuperInClosure() => () => super.a;
+}
+
+main() {
+  B b = new B();
+  Expect.equals(54, b.a);
+  Expect.equals(42, b.returnSuper());
+  Expect.equals(42, b.returnSuperInClosure()());
+}