FlarkEditRecorder class final

Records the calls made to one editor so that a session can be replayed as a test. A host attaches one while dogfooding and copies repro when something goes wrong.

The recorder observes the editor's calls (FlarkEditor.onCall) and keeps where its window starts (source, selection, forced source mode) and each call since, as the FlarkEditorCall it was, with what it returned. Its two readers of a call are repro, which writes it as Dart, and replay, which makes it again. A call keeps the change it made to the source rather than a copy of the document, and past capacity calls the oldest is folded into the window's start, so memory stays near one document plus the recent changes. What the editor held before the window starts beyond that is not replayed: its history (an Undo past the window's start does nothing in the replay), a pending style and an unwritten table cell the caret was in. The repro says when the window starts with any of them.