Add some support for the code-point code-unit distinction.

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 df0393db3f40a345623dddd1bf941d3eabc94e35..710ec3ad90d43e36b8dfde0eeeb98cae7461873d 100644
--- a/runtime/lib/string_patch.dart
+++ b/runtime/lib/string_patch.dart
@@ -3,8 +3,8 @@
 // BSD-style license that can be found in the LICENSE file.
 
 patch class String {
-  /* patch */ factory String.fromCharCodes(List<int> charCodes) {
-    return _StringBase.createFromCharCodes(charCodes);
+  /* patch */ factory String.fromCodeUnits(List<int> codeUnits) {
+    return _StringBase.createFromUtf16(codeUnits);
   }
 }