Use exact name lookup for static fields in mirrors.

tests/lib/mirrors/private_class_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.
+
+// Test a private field name doesn't match the equivalent private name from
+// another library.
+
+import 'dart:mirrors';
+import 'package:expect/expect.dart';
+
+import 'private_class_field_other.dart';
+
+void main() {
+  var classMirror = reflectClass(C);
+  // The symbol is private w/r/t the wrong library.
+  Expect.throws(() => classMirror.getField(#_privateField),
+                (e) => e is NoSuchMethodError);
+
+  Expect.equals(42, classMirror.getField(privateFieldSymbolInOther).reflectee);
+}