Strip flow syntax before any other transform (#5182)

* Strip flow syntax before any other transform
Plugins run before presets so we can't use the flow preset

* Add informative comment

Author: Timer

packages/babel-preset-react-app/create.js

@@ -95,9 +95,13 @@ module.exports = function(api, opts, env) {
           useBuiltIns: true,
         },
       ],
-      isFlowEnabled && [require('@babel/preset-flow').default],
     ].filter(Boolean),
     plugins: [
+      // Strip flow types before any other transform, emulating the behavior
+      // order as-if the browser supported all of the succeeding features
+      // https://github.com/facebook/create-react-app/pull/5182
+      isFlowEnabled &&
+        require('@babel/plugin-transform-flow-strip-types').default,
       // Experimental macros support. Will be documented after it's had some time
       // in the wild.
       require('babel-plugin-macros'),

packages/babel-preset-react-app/package.json

@@ -23,11 +23,11 @@
     "@babel/plugin-syntax-dynamic-import": "7.0.0",
     "@babel/plugin-transform-classes": "7.1.0",
     "@babel/plugin-transform-destructuring": "7.0.0",
+    "@babel/plugin-transform-flow-strip-types": "7.0.0",
     "@babel/plugin-transform-react-constant-elements": "7.0.0",
     "@babel/plugin-transform-react-display-name": "7.0.0",
     "@babel/plugin-transform-runtime": "7.1.0",
     "@babel/preset-env": "7.1.0",
-    "@babel/preset-flow": "7.0.0",
     "@babel/preset-react": "7.0.0",
     "@babel/runtime": "7.0.0",
     "babel-loader": "8.0.4",