VM: Optimized code for all of [External]{One|Two}ByteString::codeUnitAt.

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 29c473933d8d086dfeba67be46614f7850eae603..266b35a893fad456cbb4aebd8e43295f2ae51c9d 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -15531,7 +15531,8 @@ intptr_t Instance::ElementSizeFor(intptr_t cid) {
 
 
 intptr_t Instance::DataOffsetFor(intptr_t cid) {
-  if (RawObject::IsExternalTypedDataClassId(cid)) {
+  if (RawObject::IsExternalTypedDataClassId(cid) ||
+      RawObject::IsExternalStringClassId(cid)) {
     // Elements start at offset 0 of the external data.
     return 0;
   }