Support use of external strings as inputs to LoadCodeUnitsInstr.

runtime/vm/flow_graph_range_analysis.cc

Index: runtime/vm/flow_graph_range_analysis.cc
diff --git a/runtime/vm/flow_graph_range_analysis.cc b/runtime/vm/flow_graph_range_analysis.cc
index 98d3da533bd641e973103cfcf61bf0062bfb5e7a..009e3fffc36bf1814b834083a5f8d75905574bd0 100644
--- a/runtime/vm/flow_graph_range_analysis.cc
+++ b/runtime/vm/flow_graph_range_analysis.cc
@@ -2904,11 +2904,12 @@ void LoadIndexedInstr::InferRange(RangeAnalysis* analysis, Range* range) {
 
 
 void LoadCodeUnitsInstr::InferRange(RangeAnalysis* analysis, Range* range) {
-  ASSERT(class_id() == kOneByteStringCid ||
-         class_id() == kTwoByteStringCid);
+  ASSERT(RawObject::IsStringClassId(class_id()));
   switch (class_id()) {
     case kOneByteStringCid:
     case kTwoByteStringCid:
+    case kExternalOneByteStringCid:
+    case kExternalTwoByteStringCid:
       *range = Range(RangeBoundary::FromConstant(0),
                      RangeBoundary::FromConstant(kMaxUint32));
       break;