new: Transform async/await to promises for browsers.

package.json

@@ -78,6 +78,7 @@
     "@rollup/plugin-babel": "^5.2.1",
     "@rollup/plugin-commonjs": "^15.1.0",
     "@rollup/plugin-node-resolve": "^9.0.0",
+    "babel-plugin-transform-async-to-promises": "^0.8.15",
     "babel-plugin-transform-dev": "^2.0.1",
     "builtin-modules": "^3.1.0",
     "execa": "^4.0.3",

src/babel/config.ts

@@ -129,15 +129,15 @@ export function getBabelInputConfig(
 // The output config does all the transformation and downleveling through the preset-env.
 // This is handled per output since we need to configure based on target + format combinations.
 export function getBabelOutputConfig(
-  buildUnit: BuildUnit,
+  { format, platform, support }: BuildUnit,
   features: FeatureFlags,
 ): ConfigStructure {
   const plugins: PluginItem[] = [];
   const presets: PluginItem[] = [];
+  const isFuture = support !== 'legacy' && support !== 'stable';
 
   // ENVIRONMENT
 
-  const { format, platform, support } = buildUnit;
   const envOptions: PresetEnvOptions = {
     // Prefer spec compliance over speed
     spec: true,
@@ -157,13 +157,21 @@ export function getBabelOutputConfig(
 
   // Use `Object.assign` when available
   // https://babeljs.io/docs/en/babel-plugin-transform-destructuring#usebuiltins
-  if (buildUnit.support !== 'legacy' && buildUnit.support !== 'stable') {
+  if (isFuture) {
     plugins.push(
       ['@babel/plugin-transform-destructuring', { useBuiltIns: true }],
       ['@babel/plugin-proposal-object-rest-spread', { useBuiltIns: true }],
     );
   }
 
+  // Transform async/await into Promises for browsers
+  if (platform === 'browser') {
+    plugins.push([
+      'babel-plugin-transform-async-to-promises',
+      { inlineHelpers: true, target: isFuture ? 'es6' : 'es5' },
+    ]);
+  }
+
   // Support `__DEV__` shortcuts
   plugins.push(['babel-plugin-transform-dev', { evaluate: false }]);
 

yarn.lock

@@ -2335,6 +2335,11 @@ babel-plugin-jest-hoist@^26.5.0:
     "@types/babel__core" "^7.0.0"
     "@types/babel__traverse" "^7.0.6"
 
+babel-plugin-transform-async-to-promises@^0.8.15:
+  version "0.8.15"
+  resolved "https://registry.yarnpkg.com/babel-plugin-transform-async-to-promises/-/babel-plugin-transform-async-to-promises-0.8.15.tgz#13b6d8ef13676b4e3c576d3600b85344bb1ba346"
+  integrity sha512-fDXP68ZqcinZO2WCiimCL9zhGjGXOnn3D33zvbh+yheZ/qOrNVVDDIBtAaM3Faz8TRvQzHiRKsu3hfrBAhEncQ==
+
 babel-plugin-transform-dev@^2.0.1:
   version "2.0.1"
   resolved "https://registry.yarnpkg.com/babel-plugin-transform-dev/-/babel-plugin-transform-dev-2.0.1.tgz#fec5bbfb6b9576cd8413df5bd0ae7aca32b0a2d4"