LoadCodeUnitsInstr can be passed both tagged and untagged string input.

runtime/vm/intermediate_language.h

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_INTERMEDIATE_LANGUAGE_H_
 #define VM_INTERMEDIATE_LANGUAGE_H_
 
 #include "vm/allocation.h"
 #include "vm/ast.h"
 #include "vm/growable_array.h"
@@ skipping 3692 matching lines @@
     ASSERT(RawObject::IsStringClassId(class_id));
     SetInputAt(0, str);
     SetInputAt(1, index);
   }
 
   intptr_t token_pos() const { return token_pos_; }
 
   DECLARE_INSTRUCTION(LoadCodeUnits)
   virtual CompileType ComputeType() const;
 
+  virtual Representation RequiredInputRepresentation(intptr_t idx) const {
+    if (idx == 0) {
+      // The string may be tagged or untagged (for external strings).
+      return kNoRepresentation;
+    }
+    ASSERT(idx == 1);
+    return kTagged;
+  }
+
   bool IsExternal() const {
     return array()->definition()->representation() == kUntagged;
   }
 
   Value* array() const { return inputs_[0]; }
   Value* index() const { return inputs_[1]; }
   intptr_t index_scale() const { return Instance::ElementSizeFor(class_id_); }
   intptr_t class_id() const { return class_id_; }
   intptr_t element_count() const { return element_count_; }
 
@@ skipping 4270 matching lines @@
       Isolate* isolate, bool opt) const {                                     \
     UNIMPLEMENTED();                                                          \
     return NULL;                                                              \
   }                                                                           \
   void Name::EmitNativeCode(FlowGraphCompiler* compiler) { UNIMPLEMENTED(); }
 
 
 }  // namespace dart
 
 #endif  // VM_INTERMEDIATE_LANGUAGE_H_