Clean up legacy test configurations

[natebosch]

We should no longer need:

• preview-dart-2 argument or any tests against no-preview-dart-2
• Compiler.appJit, replaced with Compiler.appJitk
• Compiler.precompiled, replaced with Compiler.dartkp
• checked argument or any tests against unchecked

We do still need host-checked. It would be best to rename to host-asserts-enabled but it may not be worth changing.

[mkustermann]

Regarding we no longer need: checked argument: The --checked flag not only added type checks (which are not needed anymore), but also assertions. We want to continue checking tests with assertions enabled. Some time ago I've filed #31739 for this.

@athomas Do we already have bots running with assertions enabled?

[athomas]

There's this one https://ci.chromium.org/p/dart/builders/luci.dart.ci.sandbox/vm-kernel-checked-linux-release-x64 (it uses the --enable-asserts flag).

[natebosch]

Right, I'd expect we should be using the --enable-asserts flag

We only use checked in one place in test_matrix.json. My assumption was that I should be able to replace that with enable-asserts: true which is used more.

[athomas]

Interestingly, DDC's compiler configuration doesn't seem to have any code to wire up the checked or enable-asserts flags. I guess the intention here is to pass --enable-asserts to DDC which would be a no-op because assertions are enabled on DDC by default.

[natebosch]

Bob handled --preview-dart-2 in 89ad636