[dwds] Remove reassemble extension invocation for restarter

[srujzs]

#2584

This can and should be done in Flutter tools using callServiceExtension.

[srujzs]

Okay, I know I'm the one who opened this, but having Flutter tools invoke this might be the wrong approach for now.

To summarize:

• This is where we call service extensions. We call without checking if there exists an extension with that name - this then leads to an unexpected error in the JS evaluation as DDC throws. Flutter tools could check for this, but we don't throw a specific error for it to catch. We should throw an RPCError with kind kMethodNotFound. The Flutter VM service wrapper indeed checks for this when invoking ext.flutter.reassemble already. This allows integration tests that don't register reassemble to pass.
• We cache the present extension RPCs in a list in the initialization of the AppInspector. This list is added to over time as well. Information may become stale here.

Note that we check for the extension before we call it in the restarter. No such mechanism API exists with the VM service to query the list of extensions afaict.