[WIP] 支持 null safety

[huangbinjie]

相关 issue: #40

[codecov[bot]]

Codecov Report

Merging #43 (e1e0a5c) into master (7c06d3f) will decrease coverage by 73.13%. The diff coverage is 32.92%.

@@             Coverage Diff             @@
##           master      #43       +/-   ##
===========================================
- Coverage   89.17%   16.03%   -73.14%     
===========================================
  Files          26       27        +1     
  Lines         628      636        +8     
===========================================
- Hits          560      102      -458     
- Misses         68      534      +466     

Impacted Files	Coverage Δ
base/lib/src/error/error.dart	0.00% <0.00%> (-25.00%)	:arrow_down:
base/lib/src/storage/config/config.dart	100.00% <ø> (ø)
base/lib/src/storage/config/host.dart	0.00% <0.00%> (-97.57%)	:arrow_down:
base/lib/src/storage/error/error.dart	4.76% <0.00%> (-77.39%)	:arrow_down:
...b/src/storage/methods/put/by_part/cache_mixin.dart	0.00% <0.00%> (-100.00%)	:arrow_down:
...orage/methods/put/by_part/complete_parts_task.dart	0.00% <0.00%> (-100.00%)	:arrow_down:
...c/storage/methods/put/by_part/init_parts_task.dart	0.00% <0.00%> (-100.00%)	:arrow_down:
base/lib/src/storage/methods/put/by_part/part.dart	0.00% <ø> (-55.56%)	:arrow_down:
...orage/methods/put/by_part/put_by_part_options.dart	0.00% <ø> (-100.00%)	:arrow_down:
...rc/storage/methods/put/by_part/put_parts_task.dart	0.00% <0.00%> (-92.86%)	:arrow_down:
... and 32 more

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