New `dart` command should support experiment flags

[mit-mit]

The new dart tool should support --enable-experiment flags (needed e.g. for null safety preview)

[bkonyi]

Work in the VM is done. Let me know if you need anything else @jwren.

[jwren]

Thank you @bkonyi, I will work on getting the change into dartdev tomorrow.

[jwren]

I started this work, but it will take more time than I originally thought. For each sub-command, we'll need to reach out to each owner to either allow an API to enable the experiments or to receive the flag and enable the specified experiment:

• pub run, pub global run
• run
• test
• analyze

[mit-mit]

run seems to work for me:

$ dart run
../test_pkg/lib/test_pkg.dart:5:4: Error: This requires the 'non-nullable' language feature to be enabled.
Try updating your pubspec.yaml to set the minimum SDK constraint to 2.9 or higher, and running 'pub get'.
int? nullableBar() {
   ^

$ dart run --enable-experiment=non-nullable
Hello: null, 42, null, 42, null, -1.

[mit-mit]

While we need the flags working generally, for here and now (i.e. for null safety) I would prioritize getting test & pub (run) working; they and run are the most important.

analyze works indirectly via the flag in analysis-options.yaml.

create & format doesn't do anything wrt. null safety.

migrate is run prior to null safety being there.

[devoncarew]

@jwren - I took a stab at some of this in https://dart-review.googlesource.com/c/sdk/+/152960; the technique I'm using won't need modification of the underlying tools (hopefully, just adopting their wrapper commands).