Connect Flutter Property Editor client to DTD

[elliette]

Wire up the Flutter Property Editor to DTD so that:

• Flutter Property Editor responds toActiveLocationChangedEvent (see https://dart-review.googlesource.com/c/sdk/+/392908)
• Flutter Property Editor sends request for editable arguments to Analysis Server (see https://dart-review.googlesource.com/c/sdk/+/394160)
• Flutter Property Editor sends request for to edit an argument to Analysis Server (see https://dart-review.googlesource.com/c/sdk/+/395280)

[elliette]

Completed in:

• Connect the Property Editor to DTD  #8598
• Can send edit requests from the Property Editor #8632