Move StringImplementation from coreimpl to core as _StringImpl.

runtime/lib/string_patch.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.
 
-patch class StringImplementation {
-  /* patch */ static String _fromCharCodes(List<int> charCodes) {
+patch class _StringImpl {
+  /* patch */ factory String.fromCharCodes(List<int> charCodes) {
     return _StringBase.createFromCharCodes(charCodes);
   }
 
   /* patch */ static String join(List<String> strings, String separator) {
     return _StringBase.join(strings, separator);
   }
 
   /* patch */ static String concatAll(List<String> strings) {
     return _StringBase.concatAll(strings);
   }
 }