Debugger has several bugs when debugging a Flutter test

[kenzieschmoll]

Repro: run a flutter test with flutter test test/foo_test.dart --start-paused. Copy the VM service URI and connect to DevTools.

Bugs:

• incorrect main file is loaded
• trying to switch to a file is buggy (tried to select a file multiple times and it finally switched)
• try to switch files from the file explorer and a bunch of failed to parse exceptions are thrown

@elliette