Support use of external strings as inputs to LoadCodeUnitsInstr.

runtime/vm/symbols.h

Index: runtime/vm/symbols.h
diff --git a/runtime/vm/symbols.h b/runtime/vm/symbols.h
index bab0a9633d619eb7ff7d518420f9b5d500495744..f57b143d0fdfa8dd2d72c08fb3fdde974b5d970d 100644
--- a/runtime/vm/symbols.h
+++ b/runtime/vm/symbols.h
@@ -347,6 +347,7 @@ class ObjectPointerVisitor;
   V(match_end_index, ":match_end_index")                                       \
   V(char_in_capture, ":char_in_capture")                                       \
   V(char_in_match, ":char_in_match")                                           \
+  V(index_temp, ":index_temp")                                                 \
   V(result, ":result")                                                         \
   V(position_registers, ":position_registers")                                 \
   V(string_param, ":string_param")                                             \