Change VM's string-buffer patch to use a Uin16Array as backing buffer.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index 62929f2d4dc2e90869a5d55b3f283b9556860454..6e464d727bfec5fae76428d51c0ce9b313837876 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -5399,12 +5399,12 @@ class Uint16Array : public ByteArray {
                              intptr_t len,
                              Heap::Space space = Heap::kNew);
 
- private:
   uint8_t* ByteAddr(intptr_t byte_offset) const {
     ASSERT((byte_offset >= 0) && (byte_offset < ByteLength()));
     return reinterpret_cast<uint8_t*>(&raw_ptr()->data_) + byte_offset;
   }
 
+ private:
   FINAL_HEAP_OBJECT_IMPLEMENTATION(Uint16Array, ByteArray);
   friend class ByteArray;
   friend class Class;