Become fully Dart 2.0 compatible

[passsy]

I noticed that ktdart wasn't working in dart 2.0. Only minor changes where required to fix this.

To prevent such regressions I changed the CI setup to run on all supported dart versions

[codecov]

Codecov Report

Merging #96 into master will decrease coverage by 0.2%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
- Coverage   99.84%   99.63%   -0.21%     
==========================================
  Files          41       42       +1     
  Lines        1923     1940      +17     
==========================================
+ Hits         1920     1933      +13     
- Misses          3        7       +4

Impacted Files	Coverage Δ
...rc/collection/impl/dart_unmodifiable_set_view.dart	100% <ø> (ø)	⬆️
lib/src/collection/impl/list_empty.dart	100% <ø> (ø)	⬆️
lib/src/collection/impl/map_empty.dart	100% <ø> (ø)	⬆️
lib/src/collection/impl/map_mutable.dart	100% <ø> (ø)	⬆️
lib/src/collection/impl/set.dart	100% <ø> (ø)	⬆️
lib/src/collection/impl/set_mutable.dart	100% <ø> (ø)	⬆️
lib/src/collection/impl/map.dart	100% <ø> (ø)	⬆️
lib/src/collection/impl/set_empty.dart	100% <ø> (ø)	⬆️
lib/src/collection/impl/list.dart	100% <ø> (ø)	⬆️
lib/src/collection/impl/list_mutable.dart	100% <100%> (ø)	⬆️
... and 5 more

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