Use Image.toString instead of rolling our own each time

I meant to check this in as part of https://github.com/flutter/flutter/pull/1852
but forgot to commit the local change, d'oh.

packages/flutter/lib/src/rendering/image.dart

@@ -251,10 +251,7 @@ class RenderImage extends RenderBox {
 
   void debugDescribeSettings(List<String> settings) {
     super.debugDescribeSettings(settings);
-    if (image != null)
-      settings.add('image: [${image.width}\u00D7${image.height}]');
-    else
-      settings.add('image: null');
+    settings.add('image: $image');
     if (width != null)
       settings.add('width: $width');
     if (height != null)

packages/flutter/lib/src/services/image_resource.dart

@@ -11,7 +11,7 @@ class ImageInfo {
   ImageInfo({ this.image, this.scale: 1.0 });
   final ui.Image image;
   final double scale;
-  String toString() => '[${image.width}\u00D7${image.height}] @ ${scale}x';
+  String toString() => '$image @ ${scale}x';
 }
 
 /// A callback for when the image is available.

packages/flutter/lib/src/widgets/basic.dart

@@ -1593,10 +1593,7 @@ class RawImage extends LeafRenderObjectWidget {
 
   void debugFillDescription(List<String> description) {
     super.debugFillDescription(description);
-    if (image != null)
-      description.add('image: [${image.width}\u00D7${image.height}]');
-    else
-      description.add('image: null');
+    description.add('image: $image');
     if (width != null)
       description.add('width: $width');
     if (height != null)