Covers lib/ in package:flutter.

* Print the error message multiple times to avoid overlap with status bar and add paragraph to mock_canvas

Also, make sure that the parent is notified when they change.

Fixes #2298

Turns out there were more clients of the old paragraph API than I expected.
This patch migrates them to the new API.

We'll eventually remove Paragraph#paint.

Fixes #2694

Now a RenderBox is considered hit if one of its children are hit or it itself
decides that it's hit. In particular, empty space inside a flex won't be hit
because none of the children are located there and a RenderFlex doesn't
consider itself hittable.

Fixes #53
Fixes #1221

Now that you import a top-level file for each layer of the Flutter
framework, within the framework we can use relative paths without being
worried about the copy/pasta problem we used to have.

Almost done adding dartdoc to the rendering layer.

If the build stack got long before, it would get cropped.