add use_if_null_to_convert_null_to_bool

[a14n]

Description

Use if-null operator to convert null to bool.

BAD:

if (nullableBool == true) {
}

GOOD:

if (nullableBool ?? false) {
}

Fixes #2460

[coveralls]

Coverage increased (+0.008%) to 96.617% when pulling 5424869 on a14n:use_if_null_to_convert_null_to_bool into 087a1c6 on dart-lang:master.

[bwilkerson]

Do we want to catch cases where the operands are reversed (true == e)?
Do we want to catch uses of != (e != false)?

[a14n]

Do we want to catch cases where the operands are reversed (true == e)?

I don't want to bother yoda conditions lovers :) (this could be another lint BTW - avoid_yoda_conditions)

Do we want to catch uses of != (e != false)?

I will add some code to catch that.

[pq]

Consider renaming to use_if_null_to_convert_nulls_to_bools?

I feel like we've mostly pluralized. For example:

use_key_in_widget_constructors
use_late_for_private_fields_and_variables
use_setters_to_change_properties

etc.

[pq]

=>

'Use if-null operators to convert nulls to bools.';

(We standardized on plurals -- mostly 😬 )

[pq]

=>

Use if-null operators to convert nulls to bools.

[pq]

PS: this is super awesome!

[pq]

Looks great!

[pq]

Thanks!