Rationalize Dart mojo and sky package structure

sky/sdk/lib/framework/editing/editable_string.dart

 // Copyright 2015 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 'package:mojom/keyboard/keyboard.mojom.dart';
 
 typedef void StringUpdated();
 
 class TextRange {
   final int start;
@@ skipping 102 matching lines @@
     composing = _replaceOrAppend(composing, text);
     selection = new TextRange.collapsed(composing.end);
     onUpdated();
   }
 
   void setSelection(int start, int end) {
     selection = new TextRange(start: start, end: end);
     onUpdated();
   }
 }