Fix several problems with anonymous functions and type parameters

[jcollins-g]

Fixes #1643. @bergwerf

Anonymous functions as part of type parameters haven't worked very well in dartdoc at times, and an interesting case in #1643 actually led to crashes. Fix that and a few other related problems:

• Not everything has libraries, so refactor ModelElement.from to require both a library and package graph, and assert if at least one isn't null (root cause of Crash handling anonymous functions as keys to a Map with inferred types #1643)
• Show anonymous functions when they are type parameters
• Shift style so that we don't line break between parameters of an anonymous function used as a type parameter.

[jcollins-g]

Some screenshots:

The case from #1643 in summary view:

The case from #1643 when viewing the symbol:

Another related case, summary view:

And when viewing the symbol:

[devoncarew]

👍