Add a way to prefetch remote images to cache with Image.prefetch

Summary:Adds `Image.prefetch` to prefetch remote images before they are used in an actual `Image` component. This is based off of #4420 by sospartan and skevy's work.
Closes https://github.com/facebook/react-native/pull/6774

Differential Revision: D3153729

Pulled By: bestander

fb-gh-sync-id: ef61412e051a49b42ae885edce7905a8ca0da23f
fbshipit-source-id: ef61412e051a49b42ae885edce7905a8ca0da23f

Libraries/Image/Image.android.js

@@ -23,11 +23,14 @@ var StyleSheetPropType = require('StyleSheetPropType');
 var View = require('View');
 
 var flattenStyle = require('flattenStyle');
-var invariant = require('fbjs/lib/invariant');
 var merge = require('merge');
 var requireNativeComponent = require('requireNativeComponent');
 var resolveAssetSource = require('resolveAssetSource');
 
+var {
+  ImageLoader,
+} = NativeModules;
+
 /**
  * <Image> - A react component for displaying different types of images,
  * including network images, static resources, temporary local images, and
@@ -110,6 +113,13 @@ var Image = React.createClass({
 
   statics: {
     resizeMode: ImageResizeMode,
+    /**
+     * Prefetches a remote image for later use by downloading it to the disk
+     * cache
+     */
+    prefetch(url: string) {
+      return ImageLoader.prefetchImage(url);
+    },
   },
 
   mixins: [NativeMethodsMixin],