Added ContentProvider and FileContentProvider and setup tests.
Moved Providers over to workspace.dart and removed code for content change events.
Implemented internal onContentChange events
Removed <String> param on change event and renamed to onChange, etc
Moved providers from workspace to editors and removed onChange updater in `write` method.
Changed how onChange gets triggered and changed test.
(Most) review fixes
Moved tests to editors_test.dart
Merge pull request #3469 from dart-lang/umop-file_provider

Create providers and tests