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

[eernstg]

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

[jmesserly]

fyi, I don't think we'll have to do anything once analyzer fixes #27895, other than make sure we have a test case.

[eernstg]

Test cases: Please check out tests/language/initializing_formal_*_test.dart and tests/language/parameter_initializer5_test.dart (the former ones are test cases for this feature; the latter one used to test that the access was a compile-time error, and it has been adjusted now that it isn't an error any more).

[jmesserly]

@vsmenon I think you had a CL for this?

[vsmenon]

I had a CL to flip the flag, but it's obsoleted by Brian's flipping it on by default. I had checked the tests - they pass. Now that it's on, we should migrate them over:

https://codereview.chromium.org/2534293002/

[vsmenon]

Closing, as it's on by default. I'll land the tests on review.