Expose a function for plugins to check if async plugins are supported by the current compilation.

packages/babel-core/src/config/loading/files/configuration.js

@@ -142,6 +142,7 @@ const readConfigJS = makeStrongCache((filepath, cache) => {
       cache,
       // Expose ".env()" so people can easily get the same env that we expose using the "env" key.
       env: () => cache.using(() => getEnv()),
+      async: () => false,
     });
   } else {
     cache.forever();

packages/babel-core/src/config/option-manager.js

@@ -189,6 +189,7 @@ const loadDescriptor = makeWeakCache(
       const api = Object.assign(Object.create(context), {
         cache,
         env: () => cache.using(() => getEnv()),
+        async: () => false,
       });
 
       try {