remove window dependency

Summary:
Fixes required to be able to run Flipper in node.js:
* Adds checks if the `window`-object exists before using it, to allow running in node.
* Imports from within Flipper should directly reference the file they are requiring instead of `import from 'flipper'`. This was done in most of the places. Fixed a few occurrences where this wasn't the case. This is to prevent cyclic dependencies in node.
* shared packages (React, ReactDOM and Flipper) were exposed on the `window` before, changed this to `global` as this works in browser and node.
* Adds some missing methods to our electron stubs (used for testing and headless Flipper)

Reviewed By: passy

Differential Revision: D13786577

fbshipit-source-id: 145d560f1446e7d0bdec2acd8dd54dae983d7b36

src/dispatcher/server.js

@@ -78,7 +78,9 @@ export default (store: Store, logger: Logger) => {
     },
   );
 
-  window.addEventListener('beforeunload', () => {
-    server.close();
-  });
+  if (typeof window !== 'undefined') {
+    window.addEventListener('beforeunload', () => {
+      server.close();
+    });
+  }
 };