Add proper API for creating private symbols wrt a library.

tests/lib/mirrors/symbol_validation_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 symbol_validation_test;
+
+import 'dart:mirrors';
+import 'package:expect/expect.dart';
+
+validSymbol(String string) {
+  Expect.equals(string,
+                MirrorSystem.getName(new Symbol(string)),
+                'Valid symbol "$string" should be invertable');
+  Expect.equals(string,
+                MirrorSystem.getName(MirrorSystem.getSymbol(string)),
+                'Valid symbol "$string" should be invertable');
+}
+
+invalidSymbol(String string) {
+  Expect.throws(() => new Symbol(string),
+                (e) => e is ArgumentError,
+                'Invalid symbol "$string" should be rejected');
+  Expect.throws(() => MirrorSystem.getSymbol(string),
+                (e) => e is ArgumentError,
+                'Invalid symbol "$string" should be rejected');
+}
+
+main() {
+  ['%', '&', '*', '+', '-', '/', '<', '<<', '<=', '==', '>',
+   '>=', '>>', '[]', '[]=', '^', 'unary-', '|', '~', '~/']
+      .forEach(validSymbol);
+
+  ['foo', '_bar', 'baz.quz', 'fisk1', 'hest2fisk', 'a.b.c.d.e',
+   '\$', 'foo\$', 'bar\$bar']
+      .forEach(validSymbol);
+
+  ['6', '0foo', ',', 'S with M', '_invalid&private']
+      .forEach(invalidSymbol);
+}