Update to new versions of core libraries, for compatibility with flutter 1.2.2.

[speaking-in-code]

This is the start of a fix for #1. I think the issues impact tool_mobile and screenshots as well.

I'm fairly new to the flutter package ecosystem. Feedback on better ways to do this would be great.

[codecov]

Codecov Report

Merging #2 into master will increase coverage by 0.34%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master       #2      +/-   ##
==========================================
+ Coverage   47.80%   48.15%   +0.34%     
==========================================
  Files          20       20              
  Lines        1550     1541       -9     
==========================================
+ Hits          741      742       +1     
+ Misses        809      799      -10     

Impacted Files	Coverage Δ
lib/src/base/file_system.dart	44.68% <ø> (+6.49%)	⬆️
lib/src/base/process_manager.dart	100.00% <ø> (+90.90%)	⬆️
lib/src/cache.dart	37.56% <0.00%> (-0.50%)	⬇️
lib/src/base/utils.dart	40.79% <0.00%> (-0.21%)	⬇️
lib/src/base/context.dart	60.00% <0.00%> (ø)

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[jonjon1123]

@mmcc007 Can we get this merged and a new package pushed up to work with Flutter 1.2.2