Protect Euler against the closure circular reference bug

src/math/Euler.js

@@ -22,7 +22,7 @@ Euler.RotationOrders = [ 'XYZ', 'YZX', 'ZXY', 'XZY', 'YXZ', 'ZYX' ];
 
 Euler.DefaultOrder = 'XYZ';
 
-Euler.prototype = {
+Object.assign( Euler.prototype, {
 
 	constructor: Euler,
 
@@ -330,7 +330,7 @@ Euler.prototype = {
 
 	onChangeCallback: function () {}
 
-};
+} );
 
 
 export { Euler };