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 2704c5f1c2b15507f4a7a90c09bdc313d18d32a6..0c9335d19fb357c52fbee830300f4b66e7affe38 100644
--- a/runtime/lib/string_patch.dart
+++ b/runtime/lib/string_patch.dart
@@ -3,8 +3,12 @@
 // BSD-style license that can be found in the LICENSE file.
 
 patch class _StringImpl {
-  /* patch */ factory String.fromCharCodes(List<int> charCodes) {
-    return _StringBase.createFromCharCodes(charCodes);
+  /* patch */ factory String.fromCharCodes(List<int> codePoints) {
+    return _StringBase.createFromCharCodes(codePoints);
+  }
+
+  /* patch */ factory String.fromCodeUnits(List<int> codeUnits) {
+    return _StringBase.createFromCodeUnits(codeUnits);
   }
 
   /* patch */ static String join(List<String> strings, String separator) {