support ?shuffleListeners

[zepumph]

From phetsims/axon#215, when running with ?shuffleListeners, we get this error: assigning to responsible dev to determine if it is worth fixing:

curve-fitting
Uncaught Error: Assertion failed: the coefficient array should be 2 long but is 2
Error: Assertion failed: the coefficient array should be 2 long but is 2
    at window.assertions.assertFunction (http://localhost:8080/assert/js/assert.js:22:13)
    at Curve.getYValueAt (http://localhost:8080/curve-fitting/js/curve-fitting/model/Curve.js:83:15)
    at http://localhost:8080/curve-fitting/js/curve-fitting/model/Curve.js:95:38
    at new CurveShape (http://localhost:8080/curve-fitting/js/curve-fitting/model/CurveShape.js:39:24)
    at Curve.getShape (http://localhost:8080/curve-fitting/js/curve-fitting/model/Curve.js:95:12)
    at Curve.get shape [as shape] (http://localhost:8080/curve-fitting/js/curve-fitting/model/Curve.js:98:29)
    at updateCurve (http://localhost:8080/curve-fitting/js/curve-fitting/view/CurveNode.js:41:70)
    at TinyEmitter.emit (http://localhost:8080/axon/js/TinyEmitter.js:69:53)
    at NumberProperty._notifyListeners (http://localhost:8080/axon/js/Property.js:273:25)
    at NumberProperty.set (http://localhost:8080/axon/js/Property.js:171:14)