Respond to Ivan's code review comments from previous TBR cl.

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 44971de96ed0905b36d92ac3dd80511eac45172a..b5de62e94dc806861862d012ab3f4040502bb2f9 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -13690,7 +13690,7 @@ RawOneByteString* OneByteString::EscapeSpecialCharacters(const String& str) {
     }
     return OneByteString::raw(dststr);
   }
-  return OneByteString::New(static_cast<intptr_t>(0), Heap::kNew);
+  return OneByteString::raw(Symbols::Empty());
 }
 
 RawOneByteString* ExternalOneByteString::EscapeSpecialCharacters(
@@ -13719,7 +13719,7 @@ RawOneByteString* ExternalOneByteString::EscapeSpecialCharacters(
     }
     return OneByteString::raw(dststr);
   }
-  return OneByteString::New(static_cast<intptr_t>(0), Heap::kNew);
+  return OneByteString::raw(Symbols::Empty());
 }