Throw when specifying env property in middleware

packages/neutrino/Neutrino.js

@@ -216,6 +216,9 @@ module.exports = class Neutrino {
       // middleware type and options
       this.use(...middleware);
     } else if (isPlainObject(middleware)) {
+      if ('env' in middleware) {
+        throw new Error('Using "env" in middleware has been removed. Apply middleware conditionally instead.');
+      }
       // If middleware is an object, it could contain other middleware in
       // its "use" property. Run every item in "use" prop back through .use(),
       // plus set any options.

packages/neutrino/test/api_test.js

@@ -84,6 +84,24 @@ test('throws when legacy options.node_modules is set', t => {
   t.throws(() => api.use({ options }), /options\.node_modules has been removed/);
 });
 
+test('throws when middleware "env" is set', t => {
+  const api = new Neutrino();
+  const middleware = {
+    env: {
+      NODE_ENV: {
+        production: neutrino => {
+          neutrino.config.devtool('alpha');
+        }
+      }
+    }
+  };
+
+  api.config.devtool('beta');
+
+  t.throws(() => api.use(middleware), /"env" in middleware has been removed/);
+  t.is(api.config.get('devtool'), 'beta');
+});
+
 test('options.mains', t => {
   const api = new Neutrino();