Add flag to disable string externalization

runtime/vm/object.h

 // Copyright (c) 2012, 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_OBJECT_H_
 #define VM_OBJECT_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/utils.h"
@@ skipping 3069 matching lines @@
   bool is_unboxing_candidate() const {
     return UnboxingCandidateBit::decode(raw_ptr()->kind_bits_);
   }
   // Default 'true', set to false once optimizing compiler determines it should
   // be boxed.
   void set_is_unboxing_candidate(bool b) const {
     ASSERT(IsOriginal());
     set_kind_bits(UnboxingCandidateBit::update(b, raw_ptr()->kind_bits_));
   }
 
-  static bool IsExternalizableCid(intptr_t cid) {
-    return (cid == kOneByteStringCid) || (cid == kTwoByteStringCid);
-  }
+  static bool IsExternalizableCid(intptr_t cid);
 
   enum {
     kUnknownLengthOffset = -1,
     kUnknownFixedLength = -1,
     kNoFixedLength = -2,
   };
   // Returns false if any value read from this field is guaranteed to be
   // not null.
   // Internally we is_nullable_ field contains either kNullCid (nullable) or
   // any other value (non-nullable) instead of boolean. This is done to simplify
@@ skipping 5331 matching lines @@
 
 
 RawObject* MegamorphicCache::GetTargetFunction(const Array& array,
                                                intptr_t index) {
   return array.At((index * kEntryLength) + kTargetFunctionIndex);
 }
 
 }  // namespace dart
 
 #endif  // VM_OBJECT_H_