Move a bunch of src/ files into subdirectories.

lib/src/util/isolate_wrapper.dart

 // Copyright (c) 2015, 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 unittest.isolate_wrapper;
+library unittest.util.isolate_wrapper;
 
 import 'dart:async';
 import 'dart:isolate';
 
 import 'io.dart';
 
 // TODO(nweiz): Get rid of this when issue 6610 is fixed.
 /// This is a wrapper around an [Isolate] that supports a callback that will
 /// fire when [Isolate.exit] is called.
 ///
@@ skipping 23 matching lines @@
   void resume(Capability resumeCapability) => _inner.resume(resumeCapability);
   void setErrorsFatal(bool errorsAreFatal) =>
       _inner.setErrorsFatal(errorsAreFatal);
   String toString() => _inner.toString();
 
   void kill([int priority=Isolate.BEFORE_NEXT_EVENT]) {
     if (supportsIsolateKill) _inner.kill(priority);
     _onExit();
   }
 }