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 2704c5f1c2b15507f4a7a90c09bdc313d18d32a6..df0393db3f40a345623dddd1bf941d3eabc94e35 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 _StringImpl {
+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);
   }