Improve compatibility with dart 2

[achilleasa]

This PR fixes some issues reported by the dartanalyzer binary that ships with the dart2 SDK. These changes should allow this package to be used with dart2.

According to the discussion in #13, for flutter2 compatibility dartanalyzer must be ran with --no-implicit-casts which reports a couple more issues that can be fixed with a separate PR.

[codecov-io]

Codecov Report

Merging #14 into master will decrease coverage by 1.09%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master      #14     +/-   ##
=========================================
- Coverage   94.89%   93.79%   -1.1%     
=========================================
  Files          55       55             
  Lines        2625     2853    +228     
=========================================
+ Hits         2491     2676    +185     
- Misses        134      177     +43

Impacted Files	Coverage Δ
test/lib/encode_decode_test.dart	99.03% <ø> (ø)	⬆️
lib/src/enums/error_type.dart	100% <100%> (ø)	⬆️
test/lib/mocks/mocks.dart	47.88% <100%> (-0.39%)	⬇️
lib/src/client/impl/channel_impl.dart	96.69% <100%> (-0.15%)	⬇️
.../src/protocol/frame/impl/heartbeat_frame_impl.dart	16.66% <0%> (-83.34%)	⬇️
lib/src/protocol/messages/bindings/tx.dart	83.33% <0%> (-16.67%)	⬇️
lib/src/protocol/messages/bindings/exchange.dart	90% <0%> (-10%)	⬇️
lib/src/protocol/messages/bindings/queue.dart	91.07% <0%> (-8.93%)	⬇️
lib/src/protocol/messages/bindings/basic.dart	91.54% <0%> (-8.46%)	⬇️
lib/src/protocol/messages/bindings/connection.dart	93.65% <0%> (-6.35%)	⬇️
... and 44 more

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