Update dart2js to fix #80.

test/kernel/regression/issue_000080.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 Mixin {
   var field;
+  foo() => 87;
 }
 
-class A extends Object with Mixin, Mixin {
+class Foo extends Object with Mixin {
+  foo() => super.foo();
+  bar() => super.field;
 }
+
+main() {
+  var f = new Foo();
+  f.field = 42;
+  print(f.bar());
+}