disallow variables on shadow blocks

core/xml.js

@@ -535,6 +535,9 @@ Blockly.Xml.domToBlockHeadless_ = function(xmlBlock, workspace) {
       goog.asserts.assert(child.isShadow(),
                           'Shadow block not allowed non-shadow child.');
     }
+    // Ensure this block doesn't have any variable inputs.
+    goog.asserts.assert(block.getVars().length == 0,
+        'Shadow blocks cannot have variable fields.');
     block.setShadow(true);
   }
   return block;