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

packages/babel-plugin-transform-function-name/src/index.js

@@ -1,6 +1,9 @@
+import { declare } from "@babel/helper-plugin-utils";
 import nameFunction from "@babel/helper-function-name";
 
-export default function() {
+export default declare(api => {
+  api.assertVersion(7);
+
   return {
     visitor: {
       FunctionExpression: {
@@ -21,4 +24,4 @@ export default function() {
       },
     },
   };
-}
+});