Merge kernel closure conversion into the Dart SDK

pkg/kernel/testcases/closures/capture_this.dart

+// Copyright (c) 2016, 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.md file.
+
+class C {
+  var x;
+  m() => (v) => x = v;
+  f() => () => () => x;
+}
+
+main() {
+  C c = new C();
+  c.x = 41;
+  c.m()(42);
+  if (42 != c.x) throw "Unexpected value in c.x: ${c.x}";
+  var result = c.f()()();
+  if (42 != result) throw "Unexpected value from c.f()()(): $result";
+}