Better mirrors support for mixins and private fields

pkg/dev_compiler/test/codegen/lib/mirrors/private_field_test.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 file.
+
+library test.mixin;
+
+import 'dart:mirrors';
+
+import 'package:expect/expect.dart';
+
+import 'private_field_helper.dart';
+
+class Foo extends Bar {
+  int _field = 42;
+
+  static int _staticField = 99;
+}
+
+var privateSymbol = #_field;
+var publicSymbol = #field;
+
+main() {
+  Expect.equals(publicSymbol, publicSymbol2);
+  Expect.notEquals(privateSymbol, privateSymbol2);
+  
+  var foo = new Foo();
+  var m = reflect(foo);
+  m.setField(privateSymbol, 38);
+  Expect.equals(38, foo._field);
+  m.setField(privateSymbol2, "world");
+  Expect.equals("world", foo.field);
+  Expect.equals("world", m.getField(publicSymbol).reflectee);
+
+  var type = reflectClass(Foo);
+  Expect.equals(99, type.getField(#_staticField).reflectee);
+}