Add test of unittests.

tests/lib/mirrors/library_imports_prefixed_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.library_imports;
+library test.library_imports_prefixed;
 
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 import 'stringify.dart';
 
 import 'library_imports_prefixed.dart';
 
 test(MirrorSystem mirrors) {
   LibraryMirror prefixed = mirrors.findLibrary(#library_imports_prefixed);
   LibraryMirror a = mirrors.findLibrary(#library_imports_a);
   LibraryMirror b = mirrors.findLibrary(#library_imports_b);
   LibraryMirror core = mirrors.findLibrary(#dart.core);
 
   Expect.setEquals([a, b, core],
                    prefixed.libraryDependencies.map((dep) => dep.targetLibrary));
 
   Expect.stringEquals(
       'import dart.core\n'
       'import library_imports_a as prefixa\n'
       'import library_imports_b as prefixb\n',
       stringifyDependencies(prefixed));
 }
 
 main() {
   test(currentMirrorSystem());
 }