Fixes dart-lang/sdk#59219

This involved a sizeable refactoring, with a few improvements:

• Visit parts in order to find mutations of private fields.
• More idiomatic/readable extension getters.
• Store less fields-which-have-been-mutated. The previous impl stored all fields every mutated in a library, including fields never declared in the library. This new impl stores only local field elements.
• Do not declare local functions in a for-loop; this should allocate many fewer short-lived objects (_Closures).