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

pkg/compiler/lib/src/tree/dartstring.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 tree;
 
 /**
  * The [DartString] type represents a Dart string value as a sequence of Unicode
  * Scalar Values.
  * After parsing, any valid [LiteralString] will contain a [DartString]
@@ skipping 231 matching lines @@
         }
         _current = value;
         break;
       default:
         _current = code;
     }
     return true;
   }
 }