Large polygons do not triangulate

[pjcozzi]

From the forum:

When I draw polygons with large distance between points, Cesium throws this error:
"Uncaught DeveloperError: At least three positions are required."
or 
"Uncaught TypeError: Cannot call method 'subtract' of undefined."

My code:

    var lonLatToCartesian = function(x, y, z) {
        return ellipsoid.cartographicToCartesian(Cesium.Cartographic.fromDegrees(x, y, z));
    };

    var positions = [lonLatToCartesian(-44, -5,0), lonLatToCartesian(-100,-1,0), lonLatToCartesian(70,0,0),lonLatToCartesian(-44, -5,0)];
    var polygon = new Cesium.Polygon();
    polygon.setPositions(positions);
    primitives.add(polygon);

if I change the point (70,0,0) to (60,0,0), it works.

https://groups.google.com/forum/?fromgroups=#!topic/cesium-dev/xRqn3PQRrvQ

This is most likely because polygon triangulation happens on a tangent plane. At the least, we should try for a better error message.

[mramato]

As of the above pull, this no longer crashes, but is still incorrect. See that pull for details.

[pjcozzi]

Duplicate with #2788