Move StringImplementation from coreimpl to core as _StringImpl.

runtime/lib/string_patch.dart

Index: runtime/lib/string_patch.dart
diff --git a/runtime/lib/string_patch.dart b/runtime/lib/string_patch.dart
index b59e06a5e61824e246fb74c6f6998f79af6881a5..2704c5f1c2b15507f4a7a90c09bdc313d18d32a6 100644
--- a/runtime/lib/string_patch.dart
+++ b/runtime/lib/string_patch.dart
@@ -2,8 +2,8 @@
 // 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);
   }