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

lib/coreimpl/date.dart

 // Copyright (c) 2012, 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.
 
 class DateImplementation implements Date {
   final int millisecondsSinceEpoch;
   final bool isUtc;
 
   factory DateImplementation.fromString(String formattedString) {
     // Read in (a subset of) ISO 8601.
@@ skipping 164 matching lines @@
   external Duration get timeZoneOffset();
   external int get year();
   external int get month();
   external int get day();
   external int get hour();
   external int get minute();
   external int get second();
   external int get millisecond();
   external int get weekday();
 }