Fix getField/setField optimization for private members.

tests/lib/mirrors/hot_set_field_test.dart

+// Copyright (c) 2014, 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.hot_set_field;
+
+import 'dart:mirrors';
+import 'package:expect/expect.dart';
+
+class C {
+  var field;
+  var _field;
+}
+
+const int optimizationThreshold = 20;
+
+testPublic() {
+  var c = new C();
+  var im = reflect(c);
+
+  for (int i = 0; i < (2 * optimizationThreshold); i++) {
+    im.setField(#field, i);
+    Expect.equals(i, c.field);
+  }
+}
+
+testPrivate() {
+  var c = new C();
+  var im = reflect(c);
+
+  for (int i = 0; i < (2 * optimizationThreshold); i++) {
+    im.setField(#_field, i);
+    Expect.equals(i, c._field);
+  }
+}
+
+testPrivateWrongLibrary() {
+  var c = new C();
+  var im = reflect(c);
+  var selector = MirrorSystem.getSymbol('_field', reflectClass(Mirror).owner);
+
+  for (int i = 0; i < (2 * optimizationThreshold); i++) {
+    Expect.throws(() => im.setField(selector, i), (e) => e is NoSuchMethodError);
+  }
+}
+
+main() {
+  testPublic();
+  testPrivate();
+  testPrivateWrongLibrary();
+}