fix(DateFilter): cache DateFormat correctly

- cache named date formats (such as 'medium', 'short' and so on...)
- cache DateFormat for each locales

Closes #882
fix: Fixing analysis warnings, deprecated use of unittest/mock

Closes #886
fix(transformer): crashes in metadata extraction while in pub serve

Issue was around how the metadata extractor folds member annotations into the class annotations. It was doing in-place modifications to the AST which would then persist to the next build of the application in pub serve. Correct change is to clone the AST node which is to be modified and leave the original intact.

Closes #888
fix(transformer): Serializing execution of transformers

When executing under pub serve the order of apply for the transformers may not be the same as the phase ordering. This was causing deadlocks for the transformers sharing a resolver as the read of an asset for one transformer was invoking the apply for a different transformer, which would then deadlock waiting for the resolver.

Closes #889