Remove package:unittest from some tests

tests/lib/async/schedule_microtask3_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 run_async_test;
 
 import 'dart:async';
-import 'package:expect/expect.dart';
-import 'package:unittest/unittest.dart';
+import 'package:test/test.dart';
 
 main() {
   test("run async timer after async test", () {
     // Check that Timers don't run before the async callbacks.
     bool timerCallbackExecuted = false;
 
     Timer.run(expectAsync(() { timerCallbackExecuted = true; }));
 
     scheduleMicrotask(expectAsync(() {
-      Expect.isFalse(timerCallbackExecuted);
+      expect(timerCallbackExecuted, isFalse);
     }));
 
     scheduleMicrotask(expectAsync(() {
       // Busy loop.
       var sum = 1;
       var sw = new Stopwatch()..start();
       while (sw.elapsedMilliseconds < 5) {
         sum++;
       }
       if (sum == 0) throw "bad";  // Just to use the result.
       scheduleMicrotask(expectAsync(() {
-        Expect.isFalse(timerCallbackExecuted);
+        expect(timerCallbackExecuted, isFalse);
       }));
     }));
 
     scheduleMicrotask(expectAsync(() {
-      Expect.isFalse(timerCallbackExecuted);
+      expect(timerCallbackExecuted, isFalse);
     }));
   });
 }