Work towards https://github.com/dart-lang/sdk/issues/61386
A few distinct changes:
* Rename PluginInfo to PluginIsolate
* Move PluginInfo and PluginSession to a new library,
plugin_isolate.dart. PluginSession is _essentially_ a private class
to PluginIsolate, so it's good to co-locate them.
* Move PluginInfo tests and PluginSession tests to a new library.
* Extract out shared parent class for PluginManager tests,
PluginIsolate tests, and PluginSession tests.
Change-Id: Ib0c638b8797f51c57c07e277acdaea6d8202f957
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/446960
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Sam Rawlins
649944c76d