Unbreak accidental back-compat break with event optimization (#2222)

This was supposed to be purely additive. See here for more details:

https://github.com/MithrilJS/mithril.js/pull/1949#issuecomment-417824513

test-utils/domMock.js

@@ -364,10 +364,18 @@ module.exports = function(options) {
 						e.preventDefault = function() {
 							prevented = true
 						}
+						Object.defineProperty(e, "$defaultPrevented", {
+							configurable: true,
+							get: function () { return prevented }
+						})
 						var stopped = false
 						e.stopPropagation = function() {
 							stopped = true
 						}
+						Object.defineProperty(e, "$propagationStopped", {
+							configurable: true,
+							get: function () { return prevented }
+						})
 						e.eventPhase = 1
 						try {
 							for (var i = parents.length - 1; 0 <= i; i--) {