Fix spinning_mixed.dart, and resulting yak shave.

Fix spinning_mixed.dart, and resulting yak shave.

The initial fix was updating Image to NetworkImage, which is the new
name for the Widget that takes a URL to an image.

However, this exposed an underlying bug: slots given to
MultiChildRenderObjectWrapper children were not getting updated in the
initial build. Initially, we don't have a next sibling for any of the
children's roots, since we haven't yet synced anyone after the one
we're syncing. So we pass null. However, as soon as we build the next
one, the previous one now has an out of date slot: null means
"append", but it needs to be "insert before the newly inserted root of
the next child". So now we update the slot after each insertion.

This returned the spinning_mixed demo to its original state, but that
state was buggy: the image would expand to fit the button and push the
text out of the button. To resolve that, this patch changes how
RenderImage sizes an image that has no desired dimensions but has some
constraints that make it impossible to show the image at full size:
now, we try to maintain the aspect ratio and honour the constraints
all at the same time.

Also, if the constraints are tight we skip all that logic, because
it'd be a waste of time.

To help with the slot issue above, I needed to see the widget tree and
the render tree and compare them (to see where the nodes were getting
out of order), so I also extended Widget.toString() to dump a deep
tree of the widget hierarchy.

Also, while debugging all this I noticed we sometimes walk the tree
into nodes that are null, which causes crashes. So to avoid that I
added null-checks to certain walkChildren() functions.

R=abarth@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/984df92e923fa77361e774532f84bbfaa6d7fa70

[abarth-chromium, 2015-07-07 00:11:14]

abarth@chromium.org changed reviewers:
+ abarth@chromium.org

[abarth-chromium, 2015-07-07 00:11:15]

lgtm

https://codereview.chromium.org/1212943007/diff/1/sky/sdk/lib/rendering/box.dart
File sky/sdk/lib/rendering/box.dart (right):

https://codereview.chromium.org/1212943007/diff/1/sky/sdk/lib/rendering/box.d...
sky/sdk/lib/rendering/box.dart:1060: double maxHeight =
math.min(math.max(constraints.minHeight, constraints.maxHeight),
_image.height.toDouble());
Is this not just constraints.constrainHeight ?

https://codereview.chromium.org/1212943007/diff/1/sky/sdk/lib/rendering/box.d...
sky/sdk/lib/rendering/box.dart:1061: double width =
math.min(math.max(constraints.minWidth, constraints.maxWidth),
_image.width.toDouble());
Ditto

[Hixie, 2015-07-07 00:31:26]

Committed patchset #1 (id:1) manually as
984df92e923fa77361e774532f84bbfaa6d7fa70 (presubmit successful).