* Upgrade everything except matcher.
* Roll matcher (and test)
* Adjust tests that depend on flutter:test directly to depend on a shim
* Require use of package:test shim and remove other references to package:test

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

Instead, just store the default font sizes in statics. These statics are
constants for now, but we'll probably make them configurable at some point.

This patch start down the road of implementing text layout and painting without
the DOM. We can construct a basic paragraph consisting of a single run of text
and we can get through layout without crashing.