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.ios.js

@@ -15,23 +15,22 @@ var EdgeInsetsPropType = require('EdgeInsetsPropType');
 var ImageResizeMode = require('ImageResizeMode');
 var ImageStylePropTypes = require('ImageStylePropTypes');
 var NativeMethodsMixin = require('NativeMethodsMixin');
+var NativeModules = require('NativeModules');
 var PropTypes = require('ReactPropTypes');
 var React = require('React');
 var ReactNativeViewAttributes = require('ReactNativeViewAttributes');
-var View = require('View');
 var StyleSheet = require('StyleSheet');
 var StyleSheetPropType = require('StyleSheetPropType');
 
 var flattenStyle = require('flattenStyle');
-var invariant = require('fbjs/lib/invariant');
 var requireNativeComponent = require('requireNativeComponent');
 var resolveAssetSource = require('resolveAssetSource');
-var warning = require('fbjs/lib/warning');
 
 var {
+  ImageLoader,
   ImageViewManager,
   NetworkImageViewManager,
-} = require('NativeModules');
+} = NativeModules;
 
 /**
  * A React component for displaying different types of images,
@@ -181,7 +180,14 @@ var Image = React.createClass({
       ImageViewManager.getSize(uri, success, failure || function() {
         console.warn('Failed to get size for image: ' + uri);
       });
-    }
+    },
+    /**
+     * Prefetches a remote image for later use by downloading it to the disk
+     * cache
+     */
+    prefetch(url: string) {
+      return ImageLoader.prefetchImage(url);
+    },
   },
 
   mixins: [NativeMethodsMixin],