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

runtime/vm/bootstrap_natives.h

Index: runtime/vm/bootstrap_natives.h
diff --git a/runtime/vm/bootstrap_natives.h b/runtime/vm/bootstrap_natives.h
index 7b424b0dc92ae29c9fe23ca3bbf66596c1982ce5..1afcd561674de2885a42997285bb48f5888cb57b 100644
--- a/runtime/vm/bootstrap_natives.h
+++ b/runtime/vm/bootstrap_natives.h
@@ -71,11 +71,13 @@ namespace dart {
   V(ObjectArray_getLength, 1)                                                  \
   V(ObjectArray_copyFromObjectArray, 5)                                        \
   V(StringBase_createFromCodePoints, 1)                                        \
+  V(StringBase_createFromCodeUnits, 1)                                         \
   V(StringBase_substringUnchecked, 3)                                          \
   V(String_getHashCode, 1)                                                     \
   V(String_getLength, 1)                                                       \
   V(String_charAt, 2)                                                          \
   V(String_charCodeAt, 2)                                                      \
+  V(String_codeUnitAt, 2)                                                      \
   V(String_concat, 2)                                                          \
   V(String_toLowerCase, 1)                                                     \
   V(String_toUpperCase, 1)                                                     \