Convert String to a class.

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 07f3818027962bca0c14cbef3775c71a07cb7a1c..946488c69b3a4a2f14844c95e7459fc7b02ddad2 100644
--- a/runtime/lib/string_patch.dart
+++ b/runtime/lib/string_patch.dart
@@ -2,11 +2,13 @@
 // 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 String {
+  /* patch */ factory String.fromCharCodes(List<int> charCodes) {
     return StringBase.createFromCharCodes(charCodes);
   }
+}
 
+patch class Strings {
   /* patch */ static String join(List<String> strings, String separator) {
     return StringBase.join(strings, separator);
   }