Remove package:unittest from some tests

tests/lib/mirrors/library_uri_io_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.
 
 // Test library uri for a library read as a file.
 
 library MirrorsTest;
 
 import 'dart:mirrors';
 import 'dart:io';
-import 'package:unittest/unittest.dart';
+import 'package:test/test.dart';
 
 class Class {
 }
 
 testLibraryUri(var value, Uri expectedUri) {
   var valueMirror = reflect(value);
   ClassMirror valueClass = valueMirror.type;
   LibraryMirror valueLibrary = valueClass.owner;
   expect(valueLibrary.uri, equals(expectedUri));
 }
 
 main() {
   var mirrors = currentMirrorSystem();
   test("Test current library uri", () {
     Uri uri = Uri.base.resolveUri(Platform.script);
     testLibraryUri(new Class(), uri);
   });
 }