Add logging to help diagnose image loading failures

For debugging #801

packages/flutter/lib/base/image_resource.dart

@@ -9,11 +9,7 @@ typedef void ImageListener(sky.Image image);
 
 class ImageResource {
   ImageResource(this._futureImage) {
-    _futureImage.then((sky.Image image) {
-      _image = image;
-      _resolved = true;
-      _notifyListeners();
-    });
+    _futureImage.then(_handleImageLoaded, onError: _handleImageError);
   }
 
   bool _resolved = false;
@@ -33,10 +29,25 @@ class ImageResource {
     _listeners.remove(listener);
   }
 
+  void _handleImageLoaded(sky.Image image) {
+    _image = image;
+    _resolved = true;
+    _notifyListeners();
+  }
+
+  void _handleImageError(e, stackTrace) {
+    print('Failed to load image: $e\nStack trace: $stackTrace');
+  }
+
   void _notifyListeners() {
     assert(_resolved);
     List<ImageListener> localListeners = new List<ImageListener>.from(_listeners);
-    for (ImageListener listener in localListeners)
-      listener(_image);
+    for (ImageListener listener in localListeners) {
+      try {
+        listener(_image);
+      } catch(e) {
+        print('Image listener had exception: $e');
+      }
+    }
   }
 }

packages/flutter/lib/widgets/basic.dart

@@ -633,7 +633,9 @@ class ImageListener extends StatefulComponent {
     this.colorFilter,
     this.fit: ImageFit.scaleDown,
     this.repeat: ImageRepeat.noRepeat
-  }) : super(key: key);
+  }) : super(key: key) {
+    assert(image != null);
+  }
 
   ImageResource image;
   double width;
@@ -645,8 +647,7 @@ class ImageListener extends StatefulComponent {
   sky.Image _resolvedImage;
 
   void _handleImageChanged(sky.Image resolvedImage) {
-    if (!mounted)
-      return;
+    assert(mounted);
     setState(() {
       _resolvedImage = resolvedImage;
     });