Add proper API for creating private symbols wrt a library.

tests/lib/mirrors/private_symbol_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.
+
+library test;
+
+import 'dart:mirrors';
+import 'package:expect/expect.dart';
+
+main() {
+  LibraryMirror libcore = currentMirrorSystem().findLibrary(#dart.core).single;
+  LibraryMirror libmath = currentMirrorSystem().findLibrary(#dart.math).single;
+  LibraryMirror libtest = currentMirrorSystem().findLibrary(#test).single;
+
+  Symbol corefoo = MirrorSystem.getSymbol('foo', libcore);
+  Symbol mathfoo = MirrorSystem.getSymbol('foo', libmath);
+  Symbol testfoo = MirrorSystem.getSymbol('foo', libtest);
+
+  Expect.equals(corefoo, mathfoo);
+  Expect.equals(mathfoo, testfoo);
+  Expect.equals(testfoo, corefoo);
+
+  Expect.equals('foo', MirrorSystem.getName(corefoo));
+  Expect.equals('foo', MirrorSystem.getName(mathfoo));
+  Expect.equals('foo', MirrorSystem.getName(testfoo));
+  Expect.equals('foo', MirrorSystem.getName(#foo));
+
+  Symbol core_foo = MirrorSystem.getSymbol('_foo', libcore);
+  Symbol math_foo = MirrorSystem.getSymbol('_foo', libmath);
+  Symbol test_foo = MirrorSystem.getSymbol('_foo', libtest);
+
+  Expect.equals('_foo', MirrorSystem.getName(core_foo));
+  Expect.equals('_foo', MirrorSystem.getName(math_foo));
+  Expect.equals('_foo', MirrorSystem.getName(test_foo));
+  Expect.equals('_foo', MirrorSystem.getName(#_foo));
+
+  Expect.notEquals(core_foo, math_foo);
+  Expect.notEquals(math_foo, test_foo);
+  Expect.notEquals(test_foo, core_foo);
+
+  Expect.notEquals(corefoo, core_foo);
+  Expect.notEquals(mathfoo, math_foo);
+  Expect.notEquals(testfoo, test_foo);
+
+  Expect.equals(test_foo, #_foo);
+}