Make TreeMap cross-platform

[TomTirapani]

See issue: #3818
See Toolbox PR: xh/toolbox#747

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[lbwexler]

Is this completely driven by mask and errorMessage?

[lbwexler]

Tom and I discussed this -- Tom is going to take another pass and see if he can make mask and errorMessage cross-platform -- this would be more of a breaking change, but could give us much more bang for the buck, and would make a cross-platform version of this (and maybe other components) trivial.

[TomTirapani]

Updated this and the toolbox PR to make ErrorMessage and Mask cross-platform, and therefore not require two implementations of TreeMap / SplitMap.

Also made LoadingIndicator cross-platform as it was low-hanging fruit