Make sure fixed preventDefault can be called multiple times

Fixes #441 Interact .draggable and webcomponentsjs conflict in IE

src/ShadowDOM/wrappers/events.js

@@ -534,12 +534,17 @@
     Event.prototype.preventDefault = function() {
       if (!this.cancelable)
         return;
+
       unsafeUnwrap(this).preventDefault();
-      Object.defineProperty(this, 'defaultPrevented', {
-        get: function() {
-          return true;
-        }
-      });
+
+      if (!this.defaultPrevented) {
+        Object.defineProperty(this, 'defaultPrevented', {
+          get: function() {
+            return true;
+          },
+          configurable: true
+        });
+      }
     };
   }
 

src/WebComponents/dom.js

@@ -17,7 +17,7 @@
   if (!window.performance) {
     var start = Date.now();
     // only at millisecond precision
-    window.performance = {now: function(){ return Date.now() - start }};
+    window.performance = {now: function(){ return Date.now() - start; }};
   }
 
   // polyfill for requestAnimationFrame
@@ -64,12 +64,17 @@
       if (!this.cancelable) {
         return;
       }
+
       origPreventDefault.call(this);
-      Object.defineProperty(this, 'defaultPrevented', {
-        get: function() {
-          return true;
-        }
-      });
+
+      if (!this.defaultPrevented) {
+        Object.defineProperty(this, 'defaultPrevented', {
+          get: function() {
+            return true;
+          },
+          configurable: true
+        });
+      }
     };
   }
 

tests/WebComponents/html/dom.html

@@ -87,6 +87,8 @@
       var e = new Event('foo', {cancelable: true});
       e.preventDefault();
       assert.isTrue(e.defaultPrevented);
+      // call again, just in case
+      assert.doesNotThrow(e.preventDefault);
     });
   });
 </script>