Fix bodyless async functions (#4600)

* fix bodyless async functions (#4599)

* Do the same in the async-generator-functions transform

packages/babel-helper-remap-async-to-generator/src/index.js

@@ -152,7 +152,7 @@ function plainFunction(path: NodePath, callId: Object) {
 }
 
 export default function (path: NodePath, file: Object, helpers: Object) {
-  path.get("body").traverse(awaitVisitor, {
+  path.traverse(awaitVisitor, {
     file,
     wrapAwait: helpers.wrapAwait
   });

packages/babel-plugin-transform-async-generator-functions/src/index.js

@@ -22,7 +22,7 @@ export default function ({ types: t }) {
       Function(path, state) {
         if (!path.node.async || !path.node.generator) return;
 
-        path.get("body").traverse(yieldStarVisitor, state);
+        path.traverse(yieldStarVisitor, state);
 
         remapAsyncToGenerator(path, state.file, {
           wrapAsync: t.memberExpression(

packages/babel-plugin-transform-async-to-generator/test/fixtures/regression/4599/actual.js

@@ -0,0 +1,3 @@
+async () => await promise
+
+async () => { await promise }

packages/babel-plugin-transform-async-to-generator/test/fixtures/regression/4599/expected.js

@@ -0,0 +1,7 @@
+babelHelpers.asyncToGenerator(function* () {
+  return yield promise;
+});
+
+babelHelpers.asyncToGenerator(function* () {
+  yield promise;
+});