Rethrow erors in events

Instead of manually calling window.onerror, rethrow the first error
at the end of the dispatch. This way window.onerror gets all the
expected arguments which is important for error reporters.

Fixes #408

src/wrappers/events.js

@@ -165,12 +165,22 @@
     return getTreeScope(a) === getTreeScope(b);
   }
 
+  // pendingError is used to rethrow the first error we got during an event
+  // dispatch. The browser actually reports all errors but to do that we would
+  // need to rethrow the error asynchronously.
+  var pendingError;
+
   function dispatchOriginalEvent(originalEvent) {
     // Make sure this event is only dispatched once.
     if (handledEventsTable.get(originalEvent))
       return;
     handledEventsTable.set(originalEvent, true);
     dispatchEvent(wrap(originalEvent), wrap(originalEvent.target));
+    if (pendingError) {
+      var err = pendingError;
+      pendingError = null;
+      throw err;
+    }
   }
 
   function isLoadLikeEvent(event) {
@@ -324,10 +334,8 @@
           return false;
 
       } catch (ex) {
-        if (window.onerror)
-          window.onerror(ex.message);
-        else
-          console.error(ex, ex.stack);
+        if (!pendingError)
+          pendingError = ex;
       }
     }
 

test/js/events.js

@@ -1385,4 +1385,30 @@ test('retarget order (multiple shadow roots)', function() {
     assert.isFalse(checkbox.checked);
   });
 
+  test('window.onerror', function() {
+    var old = window.onerror;
+    var f;
+    var msg = 'Intentional error';
+    var errorCount = 0;
+
+    window.onerror = function(msg, source, lineNumber, columnNumber, error) {
+      document.removeEventListener('click', f);
+      window.onerror = old;
+      assert.isTrue(msg.indexOf(msg) >= 0);
+      assert.typeOf(source, 'string');
+      assert.typeOf(lineNumber, 'number');
+      assert.typeOf(columnNumber, 'number');
+      // error is not available in IE11.
+      errorCount++;
+    };
+
+    document.addEventListener('click', f = function(e) {
+      throw new Error(msg);
+    });
+
+    document.body.click();
+
+    assert.equal(errorCount, 1);
+  });
+
 });