* first pass

* x

* x

* address feedback

* support multiple extensions

* add convenience function, Object ⇒ dynamic, lerping

* remove not-useful comment

* fix examples/api lower sdk constraint

* remove trailing spaces

* remove another pesky trailing space

* improve lerp

* address feedback

* hide map implementation from constructor and copyWith

* use iterableproperty

* Revert "hide map implementation from constructor and copyWith"

This reverts commit a6994af0046e3c90dbc9405cac628feb5b2d3031.

* slow down sample

* make theme extension params required

* add null check

* improve documentation

* fix hashCode and operator == overrides

* modify existing tests

* remove trailing spaces

* add all tests except lerping

* fix lerping bug

* add toString to themeExtension example

* add lerping test

* assume non-nullability in example

* address feedback

* update docs

* remove trailing space

* use Map.unmodifiable