Dropdown: import button/button-group, fixed #2159

packages/dropdown/src/dropdown.vue

 <script>
   import Clickoutside from 'element-ui/src/utils/clickoutside';
   import Emitter from 'element-ui/src/mixins/emitter';
+  import ElButton from 'element-ui/packages/button';
+  import ElButtonGroup from 'element-ui/packages/button-group';
 
   export default {
     name: 'ElDropdown',
@@ -11,6 +13,11 @@
 
     directives: { Clickoutside },
 
+    components: {
+      ElButton,
+      ElButtonGroup
+    },
+
     props: {
       trigger: {
         type: String,