Migration tool should assume assertions enabled.

[stereotype441]

I found this issue while investigating #44280.

Repro steps:

• Clone fluttercommunity/get_it@dd56ba9.
• Change the \ to / in example/pubspec.yaml to work around Pubspec apparently allows \ as path separator under windows but not linux pub#2769.
• Run flutter pub upgrade.
• Run the migration tool using --skip-import-check.

Observed result:

• lib/get_it_impl.dart lines 329-332 (at the end of the method _GetItImplementation.get) are migrated like this:

    assert(instance is T,
        'Object with name $instanceName has a different type (${instanceFactory.registrationType.toString()}) than the one that is inferred (${T.toString()}) where you call it');

    return instance as T?;

• As a result, the return type of _GetItImplementation.get is marked as nullable, and consequently, the return type of GetIt.get is marked as nullable.

Expected result:

• The line assert(instance is T, ...); should be sufficient to indicate to the migration tool that instance is expected to have type T, therefore no ? should be added to the return statement.

This is yet another facet of #40566; since we're currently using flow analysis in the migration tool's edge builder, and flow analysis conservatively assumes the user might be running with assertions disabled, assertions like this aren't taken into account when building the nullability graph. But really, they should be taken into account, because the purpose of the edge builder is to build a nullability graph that reflects the user's intention about what should be nullable and what shouldn't be; and assertions are a valuable source of information about intention regardless of whether they get enabled at runtime or not.

[stereotype441]

As of 1c7fe71, the null safety migration tool has been removed from active development and retired. No further work on the tool is planned.

If you still need help, or you believe this issue has been closed in error, please feel free to reopen.