analyzer: Allow read-only access to initializing formals by default

[eernstg]

This is the analyzer specific issue for #27891; please refer to that issue for additional information.

[eernstg]

When ready, please remove the DartOptions directives from the tests,

tests/language/initializing_formal_*_test.dart
tests/language/parameter_initializer5_test.dart

such that it will be tested that the affected tools (the analyzer and dart2js) actually enable initializing formal access without the --initializing-formal-access flag. I haven't removed these directives because it would break the analyzer, but dart2js will work both with and without them as soon as https://codereview.chromium.org/2141023002/ is landed, which should be within a day.

[bwilkerson]

This is enabled by default as of https://codereview.chromium.org/2532953004/.

I have not yet updated the tests.

[eernstg]

I can see that https://codereview.chromium.org/2532953004/ was committed as 7e067a6. Thanks! I'll mark the analyzer subtask as done on #27891 now, because developers do not depend on the test updates to use this feature.

[eernstg]

@bwilkerson, please close this one when the tests have been updated (I believe that's the only missing piece---otherwise please indicate what else is missing).

[bwilkerson]

The tests were updated here: https://codereview.chromium.org/2540843002/