Support use of external strings as inputs to LoadCodeUnitsInstr.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index a46c121a7cf3bb579d20d4398589daf6731c8887..915b853962f6c2e61cb6113ef2845f1d18031f62 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -6089,6 +6089,10 @@ class ExternalOneByteString : public AllStatic {
     return raw_ptr(str)->external_data_->peer();
   }
 
+  static intptr_t external_data_offset() {
+    return OFFSET_OF(RawExternalOneByteString, external_data_);
+  }
+
   // We use the same maximum elements for all strings.
   static const intptr_t kBytesPerElement = 1;
   static const intptr_t kMaxElements = String::kMaxElements;
@@ -6165,6 +6169,10 @@ class ExternalTwoByteString : public AllStatic {
     return raw_ptr(str)->external_data_->peer();
   }
 
+  static intptr_t external_data_offset() {
+    return OFFSET_OF(RawExternalTwoByteString, external_data_);
+  }
+
   // We use the same maximum elements for all strings.
   static const intptr_t kBytesPerElement = 2;
   static const intptr_t kMaxElements = String::kMaxElements;