Move @babel/plugin-transform-flow-strip-types to test config

Author: AugustinLF

babel.config.js

@@ -12,8 +12,11 @@ module.exports = {
       },
     ],
   ],
-  plugins: [
-    '@babel/plugin-proposal-class-properties',
-    '@babel/plugin-transform-flow-strip-types',
-  ],
+  plugins: ['@babel/plugin-proposal-class-properties'],
+  env: {
+    test: {
+      // https://github.com/react-native-community/upgrade-support/issues/152
+      plugins: ['@babel/plugin-transform-flow-strip-types'],
+    },
+  },
 };