unify dart:html.{CSSMatrix, SVGMatrix}

[DartBot]

This issue was originally filed by @seaneagan

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dart:html has two interfaces to model a matrix:

CSSMatrix
SVGMatrix

these ought to be unified. see also issue #3053 and issue #3035.

[sethladd]

Removed Type-Defect label.
Added Type-Enhancement, Area-DOM, Triaged labels.

[iposva-google]

Removed Area-DOM label.
Added Area-HTML label.

[vsmenon]

Added this to the Later milestone.

[efortuna]

CSSMatrix is gone

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Added AssumedStale label.