Move core and coreimpl from corelib/ to lib/core and lib/coreimpl.

lib/coreimpl/duration_implementation.dart

 // Copyright (c) 2011, 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.
 
 // Dart core library.
 
 class DurationImplementation implements Duration {
   final int inMilliseconds;
 
   const DurationImplementation([int days = 0,
@@ skipping 54 matching lines @@
     }
     String twoDigitMinutes =
         twoDigits(inMinutes.remainder(Duration.MINUTES_PER_HOUR));
     String twoDigitSeconds =
         twoDigits(inSeconds.remainder(Duration.SECONDS_PER_MINUTE));
     String threeDigitMs =
         threeDigits(inMilliseconds.remainder(Duration.MILLISECONDS_PER_SECOND));
     return "${inHours}:${twoDigitMinutes}:${twoDigitSeconds}.${threeDigitMs}";
   }
 }