Add test for native mixin fields and fix the implementation.

tests/compiler/dart2js_native/native_mixin_field_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 native classes can use ordinary Dart classes with fields
+// as mixins.
+
+class A native "*A" {
+  var foo;
+}
+
+class B extends A with M1, M2 native "*B" {
+  var bar;
+}
+
+class M1 {
+  var baz;
+}
+
+class M2 {
+  var bar;
+  var buz;
+}
+
+A makeA() native;
+B makeB() native;
+
+void setup() native """
+function A() {this.foo='A-foo';}
+function B() {A.call(this);this.bar='B-bar';this.baz='M1-baz';}
+makeA = function(){return new A;};
+makeB = function(){return new B;};
+""";
+
+main() {
+  setup();
+  A a = makeA();
+  Expect.equals("A-foo", a.foo);
+  Expect.throws(() => a.bar, (e) => e is NoSuchMethodError);
+  Expect.throws(() => a.baz, (e) => e is NoSuchMethodError);
+  Expect.throws(() => a.buz, (e) => e is NoSuchMethodError);
+
+  B b = makeB();
+  Expect.equals("A-foo", b.foo);
+  Expect.equals("B-bar", b.bar);
+  Expect.equals("M1-baz", b.baz);
+  Expect.isNull(b.buz);
+
+  M1 m1 = new M1();
+  Expect.throws(() => m1.foo, (e) => e is NoSuchMethodError);
+  Expect.throws(() => m1.bar, (e) => e is NoSuchMethodError);
+  Expect.isNull(m1.baz);
+  Expect.throws(() => m1.buz, (e) => e is NoSuchMethodError);
+
+  M2 m2 = new M2();
+  Expect.throws(() => m2.foo, (e) => e is NoSuchMethodError);
+  Expect.isNull(m2.bar);
+  Expect.throws(() => m2.baz, (e) => e is NoSuchMethodError);
+  Expect.isNull(m2.buz);
+}