Warning is thrown when stacking on differing domains
Move warn
Using set to improve warn performance
Change warning
Merge branch 'develop' into stackedAreaWarning

Conflicts:
	plottable.d.ts
	plottable.js
	src/components/plots/abstractStackedPlot.ts
Merge pull request #1223 from palantir/stackedAreaWarning

Warning is thrown when stacking on differing domains