MAGETWO-52958: Better Semantic for UI Components

- fixed wrong processing of "extends" attribute

Author: vkorotun

app/code/Magento/Ui/Config/Reader.php

@@ -130,7 +130,6 @@ private function readFiles($fileList)
         }
 
         $definitionData = $this->definitionReader->read();
-        $output = $this->mergeDefinition($output, $definitionData);
 
         if (isset($output['attributes']['extends'])) {
             $extendsReader = $this->readerFactory->create(
@@ -145,6 +144,8 @@ private function readFiles($fileList)
             $output = array_replace_recursive($extendsData, $output);
         }
 
+        $output = $this->mergeDefinition($output, $definitionData);
+
         return $output;
     }