Allow dispatching the same event multiple times

src/wrappers/events.js

@@ -643,7 +643,12 @@
     },
     dispatchEvent: function(event) {
       var target = getTargetToListenAt(this);
-      return target.dispatchEvent_(unwrap(event));
+      var nativeEvent = unwrap(event);
+      // Allow dispatching the same event again. This is safe because if user
+      // code calls this during an existing dispatch of the same event the
+      // native dispatchEvent throws (that is required by the spec).
+      handledEventsTable.set(nativeEvent, false);
+      return target.dispatchEvent_(nativeEvent);
     }
   };
 
@@ -654,7 +659,6 @@
     forwardMethodsToWrapper(constructors, methodNames);
   }
 
-
   var originalElementFromPoint = document.elementFromPoint;
 
   function elementFromPoint(self, document, x, y) {

test/js/events.js

@@ -1277,4 +1277,43 @@ test('retarget order (multiple shadow roots)', function() {
     text.dispatchEvent(new Event('x'));
   });
 
+  test('dispatch same event object twice', function() {
+    var e = new Event('x', {bubbles: true});
+    var doc = wrap(document);
+
+    var count = 0;
+    function handler(e) {
+      count++;
+    };
+
+    doc.addEventListener('x', handler);
+
+    document.dispatchEvent(e);
+    assert.equal(count, 1);
+    document.dispatchEvent(e);
+    assert.equal(count, 2);
+    document.dispatchEvent(e);
+    assert.equal(count, 3);
+
+    doc.removeEventListener('x', handler);
+  });
+
+  test('Ensure nested dispatch is not allowed', function() {
+    var e = new Event('x', {bubbles: true});
+    var doc = wrap(document);
+
+    var count = 0;
+
+    doc.addEventListener('x', function f(e) {
+      count++;
+      assert.throws(function() {
+        doc.dispatchEvent(e);
+      });
+    });
+
+    doc.dispatchEvent(e);
+
+    assert.equal(count, 1);
+  });
+
 });