New CSSBoxWidget and lots of bug fixes

[Sub6Resources]

There's a lot going on in this branch but it's time for review.

tldr; Calculating widths/margins/padding/border is hard, so I made a new widget that replaces both ContainerSpan and StyledText. This refactor also caused a good chunk of Style code to need refactoring, so I did so, adding support for em, rem, px, %, and auto values on the margin/width/height properties.

Fixes:

• Fix BRs are not handled correctly in shrinkWrap mode #731, newline
  issue
• Aligns the baseline of inline content with the baseline of its parent flow, even if it has padding or borders
• Improved fontSize inheritance
• Auto margins now work for any block-flow element.
• auto width and height is now the default, rather than null

Enhancements:

• New CSSBoxWidget that extends MultiChildRenderObjectWidget to provide specialized layout abilities and access to child sizes during the layout phase.
• New Margin, Width, and Height classes to allow em, rem, px, auto, and % values to be used. (These extend a new Dimension class that does Unit checking to make sure only valid units are used)
• Negative margins are now allowed
• em values now used for h1-6 and p tag default margins and font sizes, which are more accurate at various font sizes.

Notable breaking change:

• Now requires Dart sdk >= Dart 2.17

[codecov]

Codecov Report

Base: 51.09% // Head: 51.44% // Increases project coverage by +0.35% 🎉

Coverage data is based on head (7581ea7) compared to base (ee9f478).
Patch coverage: 47.96% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1135      +/-   ##
==========================================
+ Coverage   51.09%   51.44%   +0.35%     
==========================================
  Files          11       17       +6     
  Lines        2018     2587     +569     
==========================================
+ Hits         1031     1331     +300     
- Misses        987     1256     +269     

Impacted Files	Coverage Δ
lib/src/anchor.dart	66.66% <0.00%> (-13.34%)	⬇️
lib/src/utils.dart	77.27% <0.00%> (+54.54%)	⬆️
lib/src/layout_element.dart	36.47% <4.00%> (-3.78%)	⬇️
lib/src/css_parser.dart	13.48% <4.59%> (-2.80%)	⬇️
lib/src/style/lineheight.dart	11.11% <11.11%> (ø)
lib/src/style/size.dart	33.33% <33.33%> (ø)
lib/src/style/fontsize.dart	56.25% <56.25%> (ø)
lib/style.dart	84.77% <58.62%> (-0.18%)	⬇️
lib/src/css_box_widget.dart	64.85% <64.85%> (ø)
lib/html_parser.dart	72.92% <74.57%> (-0.26%)	⬇️
... and 14 more

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[erickok]

This is impressive work work Matthew! I am going to take it for a spin tomorrow.

I have a bunch of experience with golden widget tests so if you like I can help with that.

[erickok]

This is working really well. It's not that it solves all issues we had rioght now (and it doesn't need to) but it's a great foundation. I tested on Android and Web and rendering looks good.

The code is looking good, evn though it's a bit tough to read through all changes. But that's okay, I'd rather have this base with a bug than the spaghetti we had right now sometimes. Good job @Sub6Resources

[erickok]

This is amazing work. Consider my comments as just thoughts/considerations/future work, as it's really solid code. Great job!

[erickok]

Just a thought: This might not be sufficient for list items that (via css) you force to a specific display: inline?

[Sub6Resources]

I'm going to push this one off as future work. We've got a whole slew of list issues that need to be resolved so I'll tackle this when I start tackling those.

[Sub6Resources]

Going to go ahead and merge this since the most recent changes were pretty trivial, unit tests passed, and my visual tests look the same before and after.