[devtools]blocked main thread events are not shown in performance overlay or performance monitoring tools

# Issue :
1. A blocked main thread does not trigger any warnings in default performance monitoring utilities provided by Flutter.

# Overview:
When an application blocks the main thread with a computation heavy operation, the user has a fully frozen screen. When the computation is completed and the main thread is unblocked, the app returns to the normal responsive state (nothing shocking here).

This "unresponsive" state is easy to detect when running/testing the application with a human in the loop. However, when using the provided programmatic testing toolchain, this "unresponsive" state does not trigger any issues on performance tests using the `traceAction`, `tracePerformance` or any `TimelineSummary` based diagnostics. This behavior is present in the web based performance tooling and the VSCode tooling as well.

# Example:
I have a small project outlining my understanding of my issue [here](https://github.com/peet0r/blocked_main).

# Things to note:
- I admit that this issue could be a result of my ignorance to a "deeper level” knowledge of Flutter testing methods. If so, I would love to help extend the Flutter docs with more performance testing content.

Beginne damit, das Szenario eines blockierten Main-Threads im verknüpften blocked_main-Beispiel zu reproduzieren, und vergleiche die Ergebnisse der Diagnosen traceAction, tracePerformance und TimelineSummary. Prüfe auch die webbasierten Performance-Tools und die VSCode-Tools; abgeschlossen ist die Aufgabe, wenn Ereignisse, die den Main-Thread blockieren, eine Warnung erzeugen oder anderweitig in den relevanten Performance-Diagnosen erscheinen.

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