[frontend_server] Expression evaluation is broken after hot restart when debugging flutter code

[annagrin]

Issue:

Expression evaluation is broken after hot restart on web platform.

Repro:

• Debug flutter hello_world example in VSCode, stop on a breakpoint in main
• Type "true" in expression evaluation box, observe result
• Click hot restart
• On breakpoint, type "true" in expression evaluation box again

Result:
Internal error: frontend server failed to compile 'true'

Expected:
true

Dart version:

Dart SDK version: 2.13.0-114.0.dev (dev)