Support use of external strings as inputs to LoadCodeUnitsInstr.

runtime/vm/regexp_assembler.h

Index: runtime/vm/regexp_assembler.h
diff --git a/runtime/vm/regexp_assembler.h b/runtime/vm/regexp_assembler.h
index 1c5d4fe5c4386ac682e71400ffe143b4c4cb1337..4a4696c40454accbd3e53f93c6e1211e3e26fd18 100644
--- a/runtime/vm/regexp_assembler.h
+++ b/runtime/vm/regexp_assembler.h
@@ -469,12 +469,10 @@ class IRRegExpMacroAssembler : public RegExpMacroAssembler {
                                      intptr_t character_count);
 
   // Returns the character within the passed string at the specified index.
-  Value* CharacterAt(Definition* index);
+  Value* CharacterAt(LocalVariable* index);
 
   // Load a number of characters starting from index in the pattern string.
-  Value* LoadCodeUnitsAt(Value* pattern,
-                         Value* index,
-                         intptr_t character_count);
+  Value* LoadCodeUnitsAt(LocalVariable* index, intptr_t character_count);
 
   // Check whether preemption has been requested.
   void CheckPreemption();
@@ -606,6 +604,7 @@ class IRRegExpMacroAssembler : public RegExpMacroAssembler {
   LocalVariable* match_end_index_;
   LocalVariable* char_in_capture_;
   LocalVariable* char_in_match_;
+  LocalVariable* index_temp_;
 
   LocalVariable* result_;