[flutter_tool] Various fixes for 'run' for Fuchsia.

[zanderso]

Description

A few things:

• Fixes the pkgctl command to remove the tool's package repo
• Makes the code that creates and deletes the package repo a little more resilient
• Adds cmx files for hello_world and flutter_gallery examples
• Stops passing the --no-link-platform flag to the Fuchsia kernel compiler. This was needed to avoid crashes with the flutter_runner most recently rolled into flutter/flutter. I suspect a similar change may be needed in dart_kernel.gni in the engine repo.

Breaking Change

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• No, this is not a breaking change.

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[zanderso]

@rmacnak-google @alexmarkov without removing this, apps crash on startup on a missing class in dart:ui. Is this expected?

[rmacnak-google]

That sounds like there's a mismatch between the platform being used to compile the app and the platform embedded in the runner's core snapshot. Is the missing class something that was recently introduced?

[alexmarkov]

1. We shouldn't pass --no-link-platform in AOT mode (kernel compiler should complain).
2. In JIT (debug) mode, the recommended way to load platform libraries is from core snapshot, and --no-link-platform will save some time and space. If app crashes when --no-link-platform is used, it means that isolate is not initialized from core snapshot, and platform libraries were loaded from kernel file. We saw similar problem with desktop embedder recently. If, for some reason, we don't want to use core snapshots then we can remove --no-link-platform, but this is less efficient compared to spawning isolates from core snapshot.

[zanderso]

FYI @iskakaushik

[zanderso]

Looks like: #44724

[iskakaushik]

Filed: #44925

[zanderso]

Added TODO to add back when #44925 is fixed.

[blasten]

how do you generate this file?

[zanderso]

There's currently no automated way to generate the cmx files. I initially copied one from the Fuchsia tree for the stocks example, then copied that one here, changing the component name.

[zanderso]

We'll likely need to generate one when we want to support Fuchsia with 'flutter create' though.

[blasten]

oh awesome!

[iskakaushik]

LGTM