Move dart2js from sdk/lib/_internal/compiler to pkg/compiler

pkg/compiler/lib/src/code_buffer.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.
 
 part of dart2js;
 
 class CodeBuffer implements StringBuffer {
 
   StringBuffer buffer = new StringBuffer();
   List<CodeBufferMarker> markers = new List<CodeBufferMarker>();
@@ skipping 109 matching lines @@
     });
   }
 }
 
 class CodeBufferMarker {
   final int offsetDelta;
   final sourcePosition;
 
   CodeBufferMarker(this.offsetDelta, this.sourcePosition);
 }