Don't allow seal or element property definition on external arrays.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index db65d5befbae7f779c1ef370fb3fa56bea3b7939..9418377c5ad059ee6b0088ab5eb7976670ac3a77 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -3414,6 +3414,15 @@ MaybeObject* JSObject::PreventExtensions() {
     return JSObject::cast(proto)->PreventExtensions();
   }
 
+  // It's not possible to seal objects with external array elements
+  if (HasExternalArrayElements()) {
+    Handle<Object> args[1] = { args[0] };
+    Handle<Object> error  =
+        isolate->factory()->NewTypeError("seal_external_array_elements",
+                                         HandleVector(args, 1));
+    return isolate->Throw(*error);
+  }
+
   // If there are fast elements we normalize.
   NumberDictionary* dictionary = NULL;
   { MaybeObject* maybe = NormalizeElements();