Two dart debug extension for different copies of an app connect to the same app

Repro:

Open the same app it two different tabs, click on dart debug extension for each of them, put a breakpoint in one of the debuggers, reload one app, then another. Only one of the reloads will actually stop in the debugger, and when it does it happens in both debuggers.

Reproduce the issue by opening the same app in two tabs, connecting the Dart debug extension to each, setting a breakpoint, and reloading both tabs. Trace how the two debugger connections are associated with app reloads. Done means each reload stops only in its own debugger.

Written by the indexing model from the issue text.