Fix build break.

vm/dart_api_impl.cc

 // 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.
 
 #include "include/dart_api.h"
 
 #include "vm/bigint_operations.h"
 #include "vm/class_finalizer.h"
 #include "vm/compiler.h"
 #include "vm/dart.h"
@@ skipping 1786 matching lines @@
     // to get at the contents.
     intptr_t copy_len = str_obj.Length();
     if (str_obj.IsOneByteString()) {
       ASSERT(length >= copy_len);
       uint8_t* latin1_array = reinterpret_cast<uint8_t*>(array);
       for (intptr_t i = 0; i < copy_len; i++) {
         latin1_array[i] = static_cast<uint8_t>(str_obj.CharAt(i));
       }
     } else {
       ASSERT(str_obj.IsTwoByteString());
-      ASSERT(length >= (copy_len * sizeof(uint16_t)));
+      ASSERT(length >= (copy_len * str_obj.CharSize()));
       uint16_t* utf16_array = reinterpret_cast<uint16_t*>(array);
       for (intptr_t i = 0; i < copy_len; i++) {
         utf16_array[i] = static_cast<uint16_t>(str_obj.CharAt(i));
       }
     }
     return Api::Null(isolate);
   }
   return Api::NewHandle(isolate,
                         str_obj.MakeExternal(array, length, peer, cback));
 }
@@ skipping 2688 matching lines @@
   }
   {
     NoGCScope no_gc;
     RawObject* raw_obj = obj.raw();
     isolate->heap()->SetPeer(raw_obj, peer);
   }
   return Api::Success(isolate);
 }
 
 }  // namespace dart