Support use of external strings as inputs to LoadCodeUnitsInstr.

runtime/vm/assembler_arm64.h

 // Copyright (c) 2014, 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_ASSEMBLER_ARM64_H_
 #define VM_ASSEMBLER_ARM64_H_
 
 #ifndef VM_ASSEMBLER_H_
 #error Do not include assembler_arm64.h directly; use assembler.h instead.
 #endif
@@ skipping 225 matching lines @@
     type_ = Reg;
     base_ = crn;
   }
 
   static OperandSize OperandSizeFor(intptr_t cid) {
     switch (cid) {
       case kArrayCid:
       case kImmutableArrayCid:
         return kWord;
       case kOneByteStringCid:
+      case kExternalOneByteStringCid:
         return kByte;
       case kTwoByteStringCid:
+      case kExternalTwoByteStringCid:
         return kHalfword;
       case kTypedDataInt8ArrayCid:
         return kByte;
       case kTypedDataUint8ArrayCid:
       case kTypedDataUint8ClampedArrayCid:
       case kExternalTypedDataUint8ArrayCid:
       case kExternalTypedDataUint8ClampedArrayCid:
         return kUnsignedByte;
       case kTypedDataInt16ArrayCid:
         return kHalfword;
@@ skipping 1693 matching lines @@
                                   Register value,
                                   Label* no_update);
 
   DISALLOW_ALLOCATION();
   DISALLOW_COPY_AND_ASSIGN(Assembler);
 };
 
 }  // namespace dart
 
 #endif  // VM_ASSEMBLER_ARM64_H_