#__PURE__

[boopathi]

Support #__PURE__ annotation for DCE and other optimizations ?

Refs:

• Tree shaking completely broken? webpack/webpack#2867
• Class declaration in IIFE considered as side effect mishoo/UglifyJS#1261
• Pure annotation in downlevel emits babel#5632

[kzc]

microsoft/TypeScript#13721

[j-f1]

AST Explorer example that uses an /** @pure */ comment to remove pure functions whose return values are unused.