Allow plugins to assert that a specific babel version has loaded the plugin. (#7450)

packages/babel-plugin-transform-flow-comments/src/index.js

@@ -1,7 +1,10 @@
+import { declare } from "@babel/helper-plugin-utils";
 import syntaxFlow from "@babel/plugin-syntax-flow";
 import { types as t } from "@babel/core";
 
-export default function() {
+export default declare(api => {
+  api.assertVersion(7);
+
   function wrapInFlowComment(path, parent) {
     let attach = path.getPrevSibling();
     let where = "trailing";
@@ -143,4 +146,4 @@ export default function() {
       },
     },
   };
-}
+});