Move StringImplementation from coreimpl to core as _StringImpl.

lib/core/string_buffer.dart

 // Copyright (c) 2011, 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.
 
 /**
  * The StringBuffer class is useful for concatenating strings
  * efficiently. Only on a call to [toString] are the strings
  * concatenated to a single String.
  */
 abstract class StringBuffer {
@@ skipping 99 matching lines @@
     _length = 0;
     return this;
   }
 
   /**
    * Returns the contents of buffer as a concatenated string.
    */
   String toString() {
     if (_buffer.length === 0) return "";
     if (_buffer.length === 1) return _buffer[0];
-    String result = StringImplementation.concatAll(_buffer);
+    String result = _StringImpl.concatAll(_buffer);
     _buffer.clear();
     _buffer.add(result);
     // Since we track the length at each add operation, there is no
     // need to update it in this function.
     return result;
   }
 
   List<String> _buffer;
   int _length;
 }