exclude async transfrom if already supported (#87)

packages/babel-preset-backpack/index.js

@@ -2,13 +2,16 @@ const path = require('path')
 
 const preset = {
   presets: [
-    [require('babel-preset-env').default, {
-      target: {
-        node: 'current'
-      },
-      // Webpack takes care of modules, so we don't have to.
-      modules: false
-    }]
+    [
+      require('babel-preset-env').default,
+      {
+        target: {
+          node: 'current'
+        },
+        // Webpack takes care of modules, so we don't have to.
+        modules: false
+      }
+    ]
   ],
   plugins: [
     // class { handleThing = () => { } }
@@ -17,40 +20,57 @@ const preset = {
     // The following two plugins use Object.assign directly, instead of Babel's
     // extends helper. Note that this assumes `Object.assign` is available.
     // { ...todo, completed: true }
-    [require.resolve('babel-plugin-transform-object-rest-spread'), {
-      useBuiltIns: true
-    }],
-
-    [require.resolve('babel-plugin-transform-regenerator'), {
-      // Async functions are converted to generators by babel-preset-env (which
-      // is based on babel-preset-latest)
-      async: false
-    }],
+    [
+      require.resolve('babel-plugin-transform-object-rest-spread'),
+      {
+        useBuiltIns: true
+      }
+    ],
 
+    [
+      require.resolve('babel-plugin-transform-regenerator'),
+      {
+        // Async functions are converted to generators by babel-preset-env (which
+        // is based on babel-preset-latest)
+        async: false
+      }
+    ],
 
     // This is so we don't need to add `babel-polyfill` to our webpack `entry`.
     // Unlike `babel-polyfill`, `babel-runtime` + the transform do not pollute
     // the global namespace. Yay.
     // @see https://medium.com/@jcse/clearing-up-the-babel-6-ecosystem-c7678a314bf3#.7j10g8yn0
-    [require.resolve('babel-plugin-transform-runtime'), {
-      helpers: false,
-      polyfill: false,
-      regenerator: true,
-      // Resolve the Babel runtime relative to the config.
-      moduleName: path.dirname(require.resolve('babel-runtime/package'))
-    }]
+    [
+      require.resolve('babel-plugin-transform-runtime'),
+      {
+        helpers: false,
+        polyfill: false,
+        regenerator: true,
+        // Resolve the Babel runtime relative to the config.
+        moduleName: path.dirname(require.resolve('babel-runtime/package'))
+      }
+    ]
   ]
 }
 
+const v = process.versions.node.split('.')
+if ((v[0] >= 7 && v[1] >= 6) || v[0] >= 8) {
+  preset.presets[0].exclude = [
+    'babel-plugin-transform-regenerator',
+    'transform-async-to-generator'
+  ]
+}
 if (process.env.NODE_ENV === 'test' || process.env.BABEL_ENV === 'test') {
   preset.plugins.push.apply(preset.plugins, [
     // We always include this plugin regardless of environment
     // because of a Babel bug that breaks object rest/spread without it:
     // https://github.com/babel/babel/issues/4851
     require.resolve('babel-plugin-transform-es2015-parameters'),
-
     // Jest needs this to work properly with import/export syntax
-    [require.resolve('babel-plugin-transform-es2015-modules-commonjs'), {loose: true}]
+    [
+      require.resolve('babel-plugin-transform-es2015-modules-commonjs'),
+      { loose: true }
+    ]
   ])
 }