Allow creation of public symbols using MirrorSystem.getSymbol when passing a null 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';
 
 typedef int _F(int);
@@ skipping 11 matching lines @@
   // Test private symbols are distinct across libraries, and the same within a
   // library when created multiple ways. Test the string can be properly
   // extracted.
   LibraryMirror libcore = currentMirrorSystem().findLibrary(#dart.core);
   LibraryMirror libmath = currentMirrorSystem().findLibrary(#dart.math);
   LibraryMirror libtest = currentMirrorSystem().findLibrary(#test);
 
   Symbol corefoo = MirrorSystem.getSymbol('foo', libcore);
   Symbol mathfoo = MirrorSystem.getSymbol('foo', libmath);
   Symbol testfoo = MirrorSystem.getSymbol('foo', libtest);
+  Symbol nullfoo1 = MirrorSystem.getSymbol('foo');
+  Symbol nullfoo2 = MirrorSystem.getSymbol('foo', null);
 
   Expect.equals(corefoo, mathfoo);
   Expect.equals(mathfoo, testfoo);
   Expect.equals(testfoo, corefoo);
+  Expect.equals(nullfoo1, corefoo);
+  Expect.equals(nullfoo2, 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));
+  Expect.equals('foo', MirrorSystem.getName(nullfoo1));
+  Expect.equals('foo', MirrorSystem.getName(nullfoo2));
 
   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));
 
@@ skipping 67 matching lines @@
   Expect.throws(() => MirrorSystem.getSymbol('_private'),
                 (e) => e is ArgumentError);
 
   var notALibraryMirror = 7;
   Expect.throws(() => MirrorSystem.getSymbol('_private', notALibraryMirror),
                 (e) => e is ArgumentError || e is TypeError);
 
   Expect.throws(() => MirrorSystem.getSymbol('public', notALibraryMirror),
                 (e) => e is ArgumentError || e is TypeError);
 }