Rationalize Dart mojo and sky package structure

mojo/dart/test/ping_pong_test.dart

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'dart:async';
 import 'dart:isolate';
 import 'dart:typed_data';
 
-import 'package:mojo/public/dart/core.dart' as core;
+import 'package:mojo/core.dart' as core;
 
 ByteData byteDataOfString(String s) {
   return new ByteData.view((new Uint8List.fromList(s.codeUnits)).buffer);
 }
 
 String stringOfByteData(ByteData bytes) {
   return new String.fromCharCodes(bytes.buffer.asUint8List().toList());
 }
 
 void expectStringFromEndpoint(
@@ skipping 52 matching lines @@
         eventStream.enableReadEvents();
       } else if (mojoSignals.isPeerClosed) {
         throw 'This end should close first.';
       } else {
         throw 'Unexpected event.';
       }
     });
     eventStream.enableWriteEvents();
   });
 }