Fix a crash during execution context detection

[annagrin]

Tolerate failure to detect a dart execution context ID:

• make ExecutionContext.id return null if ID is not found instead of throwing
• remember already processed contexts and retry them next time ExecutionContext.id is called
  • succeeds in case injected client is loaded later
• add tests

Closes: #2284

[elliette]

I might be misunderstanding the issue, but I wonder if we could use a completer to notify when the injected client has loaded, and await it being loaded before we try to get the execution context during a hot restart.