Background images

[anttileppa]

Adds basic support for using background images

[daohoangson]

This is great!

I wanted to give you a quick heads-up that I'm currently in the process of getting v0.14 ready for release, and it's been quite a journey so far (take a look at #897). It might take a little while to review this PR and #1021. After we merge the new version, there might be some conflicts, but no worries! The underlying architecture is still quite similar, and I'm here to lend a hand in ironing out any kinks. Thank you for your contributions!

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (25d7213) 99.53% compared to head (568bb7c) 99.51%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1057      +/-   ##
==========================================
- Coverage   99.53%   99.51%   -0.02%     
==========================================
  Files          69       70       +1     
  Lines        4044     4118      +74     
==========================================
+ Hits         4025     4098      +73     
- Misses         19       20       +1     

Files	Coverage Δ
...es/core/lib/src/internal/ops/style_background.dart	100.00% <100.00%> (ø)
packages/core/lib/src/core_widget_factory.dart	99.77% <91.66%> (-0.23%)	⬇️

... and 5 files with indirect coverage changes

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[daohoangson]

Looking great. It just needs a conflict resolve with the master branch.

[anttileppa]

Merged master and resolved review comments

[daohoangson]

Thank you for the resolve. I have merged this PR!