Update dart2js to fix #81.

test/kernel/regression/issue_000081.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 Base {
+  int hashCode = 42;
+}
+
+class Sub extends Base {
+  int _hashCode = null;
+
+  get hashCode => _hashCode ??= super.hashCode;
+
+  foo() {
+    _hashCode ??= super.hashCode;
+  }
+}
+
+main() {
+  print(new Sub().hashCode);
+  var l = [null];
+  l[0] ??= "fisk";
+}