Images in Stocks drawer layout at crazy sizes

Now that we get sky.Image callbacks synchronously, we were actually figuring in
the width and height of images the second time the drawer opened in the Stocks
app. That exposed a bug in our RenderImage layout code whereby it would ignore
the _width and _height properties when the image was non-null.

packages/flutter/lib/rendering/box.dart

@@ -1385,6 +1385,9 @@ class RenderImage extends RenderBox {
   }
 
   Size _sizeForConstraints(BoxConstraints constraints) {
+    if (constraints.isTight)
+      return constraints.constrain(Size.zero);
+
     // If there's no image, we can't size ourselves automatically
     if (_image == null) {
       double width = _width == null ? 0.0 : _width;
@@ -1392,7 +1395,6 @@ class RenderImage extends RenderBox {
       return constraints.constrain(new Size(width, height));
     }
 
-    if (!constraints.isTight) {
     // If neither height nor width are specified, use inherent image
     // dimensions. If only one dimension is specified, adjust the
     // other dimension to maintain the aspect ratio. In both cases,
@@ -1411,17 +1413,20 @@ class RenderImage extends RenderBox {
         }
         return constraints.constrain(new Size(width, height));
       }
+
       // Determine width from height
       double width = _height * _image.width / _image.height;
       return constraints.constrain(new Size(width, height));
     }
+
     if (_height == null) {
       // Determine height from width
       double height = _width * _image.height / _image.width;
       return constraints.constrain(new Size(width, height));
     }
-    }
-    return constraints.constrain(new Size(_image.width.toDouble(), _image.height.toDouble()));
+
+    assert(_width != null && _height != null);
+    return constraints.constrain(new Size(_width, _height));
   }
 
   double getMinIntrinsicWidth(BoxConstraints constraints) {