Flutter/Dart 2 compatibility changes

[rmathias86]

Flutter/Dart 2 compatibility changes

• Updating logging version due Flutter dependency requiring version 11+
• Some dart 2 support, type enforcement: Passing type argument. At least seems like working well with flutter beta/master channel.

[codecov-io]

Codecov Report

Merging #15 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #15   +/-   ##
=======================================
  Coverage   93.79%   93.79%           
=======================================
  Files          55       55           
  Lines        2853     2853           
=======================================
  Hits         2676     2676           
  Misses        177      177

Impacted Files	Coverage Δ
lib/src/client/impl/channel_impl.dart	96.69% <100%> (ø)	:arrow_up:
lib/src/client/impl/exchange_impl.dart	100% <100%> (ø)	:arrow_up:
lib/src/client/impl/consumer_impl.dart	100% <100%> (ø)	:arrow_up:
lib/src/client/impl/queue_impl.dart	100% <100%> (ø)	:arrow_up:

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