Morph the APIs for Node, ParentNode, and Element closer to the specs

sky/engine/tonic/dart_converter.h

Index: sky/engine/tonic/dart_converter.h
diff --git a/sky/engine/tonic/dart_converter.h b/sky/engine/tonic/dart_converter.h
index 381af62d658ed695dffe2463b097070723c7eac5..2982589da56a95b3703c248bfb7d12825e1b16a8 100644
--- a/sky/engine/tonic/dart_converter.h
+++ b/sky/engine/tonic/dart_converter.h
@@ -248,9 +248,10 @@ struct DartConverter<Vector<T>> {
     Dart_ListLength(handle, &length);
     result.reserveCapacity(length);
     for (intptr_t i = 0; i < length; ++i) {
-      Dart_Handle element = Dart_ListGetAt(handle, i);
-      DCHECK(!Dart_IsError(element));
-      result.append(DartConverter<T>::FromDart(element));
+      Dart_Handle item = Dart_ListGetAt(handle, i);
+      DCHECK(!Dart_IsError(item));
+      DCHECK(item);
+      result.append(DartConverter<T>::FromDart(item));
     }
     return result;
   }