This reverts commit 641f4e24.

* Check for bad characters in path on Windows build

* Doc comments

* Fix formatting

* Commenting/formatting

* Link to issue in comment

* Comments, formatting

* Global var rename

* Use persistent hash map to store _inheritedWidgets

Instead of using a HashMap and copying it down the tree
which leads to quadratic time and space complexity
use a persistent data structure which can amortize
the cost by sharing parts of the structure.

The data shows HAMT based PersistentHashMap to be
5-10x faster for building _inheritedWidgets and
considerably more space effecient (e.g. bringing
amount of memory allocated when constructing
_inheritedWidgets in a tree with 150 InheritedWidget
down to 70Kb from 970Kb).

PersistentHashMap is slower than HashMap for
access: 2-3x in relative terms, but in absolute
terms we are only talking about ~0.2ns slow down
per access and various app benchmarks we run have
have not revealed any significant regressions.

* wip

* update documentation

* x

* remove trailing spaces

* x

* remove useless CupertinoIconThemeData copyWith override

* add tests

* remove trailing spaces

* fix isConcrete

* x

* x

* x

* remove trailing spaces

* tweak docs

* mention that font filenames often indicate the supported axes

* add back cupertino IconThemeData copyWith

* update copyWith