Rename Image.fromNetwork and Image.fromAssetBundle (#5149)

These now have sorter names to make the callers less verbose.
diff --git a/examples/flutter_gallery/lib/demo/cards_demo.dart b/examples/flutter_gallery/lib/demo/cards_demo.dart
index 024036a..17380ff 100644
--- a/examples/flutter_gallery/lib/demo/cards_demo.dart
+++ b/examples/flutter_gallery/lib/demo/cards_demo.dart
@@ -66,8 +66,8 @@
                     top: 0.0,
                     bottom: 0.0,
                     right: 0.0,
-                    child: new Image.fromAssetBundle(
-                      name: destination.assetName,
+                    child: new Image.asset(
+                      destination.assetName,
                       fit: ImageFit.cover
                     )
                   ),