babel/packages/babel-plugin-transform-async-to-module-method/src/index.js

import remapAsyncToGenerator from "babel-helper-remap-async-to-generator";
import syntaxAsyncFunctions from "babel-plugin-syntax-async-functions";

import { addNamed } from "babel-helper-module-imports";

export default function({ types: t }) {
  return {
    inherits: syntaxAsyncFunctions,

    visitor: {
      Function(path, state) {
        if (!path.node.async || path.node.generator) return;

        const { module, method } = state.opts;

        let wrapAsync = state.methodWrapper;
        if (wrapAsync) {
          wrapAsync = t.cloneDeep(wrapAsync);
        } else {
          wrapAsync = state.methodWrapper = addNamed(path, method, module);
        }

        remapAsyncToGenerator(path, state.file, {
          wrapAsync,
        });
      },
    },
  };
}