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/utils/icons.js

@@ -96,7 +96,9 @@ export function getIconUrl(
     }
   }
 
-  let requestedScale: number = window.devicePixelRatio;
+  let requestedScale: number =
+    typeof window !== 'undefined' ? window.devicePixelRatio : 1;
+
   if (!SCALE.includes(requestedScale)) {
     // find the next largest size
     const possibleScale: ?number = SCALE.find(scale => {