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

packages/babel-plugin-syntax-flow/src/index.js

@@ -1,7 +1,11 @@
-export default function() {
+import { declare } from "@babel/helper-plugin-utils";
+
+export default declare(api => {
+  api.assertVersion(7);
+
   return {
     manipulateOptions(opts, parserOpts) {
       parserOpts.plugins.push("flow");
     },
   };
-}
+});