Set BUNDLE_SCRIPT's GCC output format to ES5

This removes the automatic 'use strict' directive, which we don't need.

Author: acdlite

scripts/rollup/build.js

@@ -382,6 +382,12 @@ function getPlugins(
           // https://github.com/facebook/react/issues/10909
           assume_function_wrapper: !isUMDBundle,
           renaming: !shouldStayReadable,
+          language_out:
+            bundleType === BROWSER_SCRIPT
+              ? // Setting it to ES5 removes the automatic 'use strict'
+                'ECMASCRIPT5'
+              : // TODO: What should the default output format be?
+                null,
         })
       ),
     // HACK to work around the fact that Rollup isn't removing unused, pure-module imports.