add 'whitelist' option (#31)

Ref https://github.com/babel/babel-preset-env/issues/16

experimental/babel-preset-env/README.md

@@ -59,6 +59,8 @@ Currently: "chrome, edge, firefox, safari, ie, node".
 * `modules` - Enable transformation of ES6 module syntax to another module type (Enabled by default to `"commonjs"`).
   * Can be `false` to not transform modules, or one of `["amd", "umd", "systemjs", "commonjs"]`.
 * `debug` (boolean) - `console.log` out the targets and plugins being used as well as the version specified in `/data/plugins.json`.
+* `whitelist` (Array<string>) - Enable a whitelist of plugins to always include. (Defaults to `[]`)
+  * Useful if there is a bug in a native implementation, or a combination of a non-supported feature + a supported one doesn't work. (Ex: Node 4 supports native classes but not spread) 
 
 ```js
 {
@@ -168,6 +170,30 @@ transform-async-to-generator {}
 syntax-trailing-function-commas {}
 ```
 
+### Example with `whitelist`
+
+```js
+// target chrome 52 with whitelist on arrow functions
+{
+  "presets": [
+    ["env", {
+      "targets": {
+        "chrome": 52
+      },
+      "whitelist": ["transform-es2015-arrow-functions"]
+    }]
+  ]
+}
+
+Using plugins:
+
+transform-exponentiation-operator {}
+transform-async-to-generator {}
+syntax-trailing-function-commas {}
+transform-es2015-arrow-functions {}
+```
+
+
 ## Caveats
 
 ### Using `babel-plugin-transform-object-rest-spread` and targeting node.js 6.5 or higher
@@ -176,4 +202,21 @@ You may get a `SyntaxError: Unexpected token ...` error if using the [object-res
 
 This is a known issue at [babel/babel#4074](https://github.com/babel/babel/issues/4074).
 
-A simple workaround would be to re-enable the following plugins: `babel-plugin-transform-es2015-destructuring` and `babel-plugin-transform-es2015-parameters`.
+A simple workaround would be to re-enable the following plugins: `babel-plugin-transform-es2015-destructuring` and `babel-plugin-transform-es2015-parameters`, which can be done with the `whitelist` option.
+
+```js
+{
+  "presets": [
+    ["env", {
+      "targets": {
+        "node": 6.5
+      },
+      "whitelist": [
+        "transform-es2015-destructuring",
+        "transform-es2015-parameters"
+      ]
+    }]
+  ],
+  "plugins": ["transform-object-rest-spread"]
+}
+```

experimental/babel-preset-env/scripts/build-data.js

@@ -74,7 +74,7 @@ const getLowestImplementedVersion = ({ features }, env) => {
   .map(({ res: test, name }, i) => {
     return Object.keys(test)
     .filter((t) => t.startsWith(env))
-    // TODO: make flagged/etc an options
+    // Babel assumes strict mode
     .filter((test) => tests[i].res[test] === true || tests[i].res[test] === "strict")
     // normalize some keys
     .map((test) => envMap[test] || test)

experimental/babel-preset-env/src/index.js

@@ -93,6 +93,26 @@ export const validateModulesOption = (modulesOpt = "commonjs") => {
   return modulesOpt;
 };
 
+export const validateWhitelistOption = (whitelistOpt = []) => {
+  if (!Array.isArray(whitelistOpt)) {
+    throw new Error(`The 'whitelist' option must be an Array<string> of plugins
+      {
+        "presets": [
+          ["env", {
+            "targets": {
+              "chrome": 50
+            },
+            "whitelist": ["transform-es2015-arrow-functions"]
+          }]
+        ]
+      }
+      was passed "${whitelistOpt}" instead
+    `);
+  }
+
+  return whitelistOpt;
+};
+
 export default function buildPreset(context, opts) {
   if (!opts.targets) {
     throw new Error(
@@ -112,6 +132,7 @@ babel-preset-env requires a "targets" option:
 
   const loose = validateLooseOption(opts.loose);
   const moduleType = validateModulesOption(opts.modules);
+  const whitelist = validateWhitelistOption(opts.whitelist);
   const targets = getTargets(opts.targets);
   const debug = opts.debug;
 
@@ -136,7 +157,7 @@ babel-preset-env requires a "targets" option:
     });
   }
 
-  transformations = transformations.map(pluginName => {
+  transformations = [...transformations, ...whitelist].map(pluginName => {
     return [require(`babel-plugin-${pluginName}`), { loose }];
   });
 

experimental/babel-preset-env/test/fixtures/preset-options/whitelist/actual.js

@@ -0,0 +1 @@
+import a from 'a';

experimental/babel-preset-env/test/fixtures/preset-options/whitelist/expected.js

@@ -0,0 +1,7 @@
+'use strict';
+
+var _a = require('a');
+
+var _a2 = _interopRequireDefault(_a);
+
+function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { default: obj }; }

experimental/babel-preset-env/test/fixtures/preset-options/whitelist/options.json

@@ -0,0 +1,9 @@
+{
+  "presets": [
+    ["../../../../lib", {
+      "targets": {},
+      "whitelist": ["transform-es2015-modules-commonjs"],
+      "modules": false
+    }]
+  ]
+}