Have dispatch return a value

[andrasferenczi]

This is related to #26.

A return value has been added to the dispatch and all related middleware functions. These are necessary to be able to be able to await the result of an async process (as asked in this issue on how to redux_thunk).

dynamic return value has the following advantages:

• does not break existing API (Minor version change seems to be enough)
• middleware return values can be chained

Dart's type system is not complex enough (as opposed to TypeScript's) to be able to write union types when you want to chain together a middleware that may return an object of type A and another that may return an object of type B. So I don't see any other meaningful way to implement return values other than dynamic.

@johnpryan Hope this can be merged soon. Thanks.

[brianegan]

Hey, thanks so much! We're working on version 4.x and would love to include this change. Great work!

[codecov-io]

Codecov Report

Merging #52 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master     #52   +/-   ##
======================================
  Coverage    92.1%   92.1%           
======================================
  Files           2       2           
  Lines          38      38           
======================================
  Hits           35      35           
  Misses          3       3

Impacted Files	Coverage Δ
lib/src/utils.dart	100% <100%> (ø)	⬆️
lib/src/store.dart	88% <100%> (ø)	⬆️

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