Import JSON from dart:convert.
Remove unused imports.
Fix context2d method call.
Use page/client.x/y instead of page/clientX/Y everywhere.
Add missing null checks.
JSON fix.
Avoid return warning.
Avoid dart2js confusion between namespace and local varialbe "gl".
Replace faulty factory constructors by static factory methods.
Remove unsafe check whether v is double.
Improve getTangent a bit.
Avoid "unnecessary cast" warnings.
Avoid dart2js warnings.
Fix Canvas_Geometry_Cube for WebGLRenderer.