VM Tools page has several bugs when debugging a Flutter test

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

bugs:
- data doesn't load on initial selection of vm tabs. If it is waiting on something, there is no indicator
- switching isolates or clicking refresh throws exceptions
- the page feels generally broken, difficult to trust the state

@bkonyi