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

static/electron-stubs.notjs

@@ -5,11 +5,25 @@
     },
     getCurrentWindow: function() {
       return {
-        isFocused: function() {return true;}
+        isFocused: function() {return true;},
+        on: function() {return true;}
       };
     },
     app: {
-      getVersion: function() {return '1';}
+      getVersion: function() {return '1';},
+      getName: function() {return '';}
+    },
+    shell: {
+      openExternal: function() {}
+    },
+    Menu: {
+      buildFromTemplate: function() {
+        return {items: []}
+      },
+      setApplicationMenu: function() {}
     }
   },
+  ipcRenderer: {
+    on: function() {return true;}
+  },
 }