Pass Systrace and Refresh as globals

Summary:
The `Systrace` and `Refresh` dependencies are injected into the `metroRequire` implementation by assigning the values to e.g. `require.Systrace = ...`.

The issue with this approach is that some `require` implementations might not support extending the `require` object or doing so results in a degraded performance. An example where this is the case is Hermes where changing the `require` object forces Hermes to opt out of the static require optimization.

This diff extends Metro so that the `Systrace` and `Refresh` implementation can either be injected by assigning to `require.Systrace` or by exposing the implementation in the global scope. It further changes the `Systrace` and `Refresh` modules to inject the instances using the global scope instead of extending `require`.

Changelog:
[Internal][Changed] - Expose Systrace and ReactRefresh as globals instead of extending require.

Reviewed By: motiz88

Differential Revision: D25693381

fbshipit-source-id: 254d66d43e7a56d3310cf1a17d5146b8d1307562

Libraries/Core/setUpReactRefresh.js

@@ -45,5 +45,7 @@ if (__DEV__) {
     },
   };
 
-  (require: any).Refresh = Refresh;
+  // The metro require polyfill can not have dependencies (applies for all polyfills).
+  // Expose `Refresh` by assigning it to global to make it available in the polyfill.
+  global[(global.__METRO_GLOBAL_PREFIX__ || '') + '__ReactRefresh'] = Refresh;
 }

Libraries/Performance/Systrace.js

@@ -209,11 +209,9 @@ const Systrace = {
 };
 
 if (__DEV__) {
-  // This is needed, because require callis in polyfills are not processed as
-  // other files. Therefore, calls to `require('moduleId')` are not replaced
-  // with numeric IDs
-  // TODO(davidaurelio) Scan polyfills for dependencies, too (t9759686)
-  (require: $FlowFixMe).Systrace = Systrace;
+  // The metro require polyfill can not have dependencies (true for all polyfills).
+  // Ensure that `Systrace` is available in polyfill by exposing it globally.
+  global[(global.__METRO_GLOBAL_PREFIX__ || '') + '__SYSTRACE'] = Systrace;
 }
 
 module.exports = Systrace;