Allow null as KtList.of() argument

[passsy]

Allow null as valid argument for listOf and setOf functions and KtList.of and KtSet.of constructors.

The hack, I first saw in freezed by @rrousselGit, splits definition and implementation of a function, allowing the detection of null values.

The definition uses typesafe T arguments and forwards to the implementation which flexes the type to Object with an internal known defaultArgument.

// definition
KtList<T> Function<T>([T arg0,T arg1]) listOf = _listOf;

// implementation with default values to detect `null`
KtList<T> _listOf<T>([Object arg0 = defaultArgument, Object arg1 = defaultArgument]) {
  return KtList.from([
    if (arg0 != defaultArgument) arg0 as T,
    if (arg1 != defaultArgument) arg1 as T,
  ]);
}

It is typeSafe from the outside, nobody is able to call the private _listOf function.

[codecov]

Codecov Report

Merging #124 into master will decrease coverage by 0.02%.
The diff coverage is 99.33%.

@@            Coverage Diff             @@
##           master     #124      +/-   ##
==========================================
- Coverage   99.64%   99.61%   -0.03%     
==========================================
  Files          38       38              
  Lines        1972     2080     +108     
==========================================
+ Hits         1965     2072     +107     
- Misses          7        8       +1     

Flag	Coverage Δ
#unittests	99.61% <99.33%> (-0.03%)	⬇️

Impacted Files	Coverage Δ
lib/src/util/arguments.dart	0.00% <0.00%> (-100.00%)	⬇️
lib/src/collection/collections.dart	100.00% <100.00%> (ø)
lib/src/collection/kt_list.dart	100.00% <100.00%> (ø)
lib/src/collection/kt_list_mutable.dart	100.00% <100.00%> (ø)
lib/src/collection/kt_set.dart	100.00% <100.00%> (ø)
lib/src/collection/kt_set_hash.dart	100.00% <100.00%> (ø)
lib/src/collection/kt_set_linked.dart	100.00% <100.00%> (ø)
lib/src/collection/kt_set_mutable.dart	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cd1f4d9...1fb8132. Read the comment docs.