Support use of external strings as inputs to LoadCodeUnitsInstr.

runtime/vm/intermediate_language_arm64.cc

Index: runtime/vm/intermediate_language_arm64.cc
diff --git a/runtime/vm/intermediate_language_arm64.cc b/runtime/vm/intermediate_language_arm64.cc
index 47f908907f79f49d612154cde2a3d6cffb0746b4..fedbd340f9e7ad6df3a9273cfb441a62132c29d1 100644
--- a/runtime/vm/intermediate_language_arm64.cc
+++ b/runtime/vm/intermediate_language_arm64.cc
@@ -903,9 +903,14 @@ LocationSummary* LoadUntaggedInstr::MakeLocationSummary(Isolate* isolate,
 
 
 void LoadUntaggedInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
-  const Register object = locs()->in(0).reg();
+  const Register obj = locs()->in(0).reg();
   const Register result = locs()->out(0).reg();
-  __ LoadFieldFromOffset(result, object, offset(), PP);
+  if (object()->definition()->representation() == kUntagged) {
+    __ LoadFromOffset(result, obj, offset(), PP);
+  } else {
+    ASSERT(object()->definition()->representation() == kTagged);
+    __ LoadFieldFromOffset(result, obj, offset(), PP);
+  }
 }
 
 
@@ -1149,11 +1154,12 @@ LocationSummary* LoadCodeUnitsInstr::MakeLocationSummary(Isolate* isolate,
 
 
 void LoadCodeUnitsInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
-  const Register array = locs()->in(0).reg();
+  // The string register points to the backing store for external strings.
+  const Register str = locs()->in(0).reg();
   const Location index = locs()->in(1);
 
   Address element_address = __ ElementAddressForRegIndex(
-        true,  IsExternal(), class_id(), index_scale(), array, index.reg());
+        true,  IsExternal(), class_id(), index_scale(), str, index.reg());
   // Warning: element_address may use register TMP as base.
 
   Register result = locs()->out(0).reg();