Add constructors to all the event classes.

client/html/src/OverflowEventWrappingImplementation.dart

Index: client/html/src/OverflowEventWrappingImplementation.dart
diff --git a/client/html/src/OverflowEventWrappingImplementation.dart b/client/html/src/OverflowEventWrappingImplementation.dart
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+++ b/client/html/src/OverflowEventWrappingImplementation.dart
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+// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+class OverflowEventWrappingImplementation extends EventWrappingImplementation implements OverflowEvent {
+  OverflowEventWrappingImplementation._wrap(ptr) : super._wrap(ptr);
+
+  factory OverflowEventWrappingImplementation(int orient,
+      bool horizontalOverflow, bool verticalOverflow) {
+    final e = dom.document.createEvent("OverflowEvent");
+    e.initOverflowEvent(orient, horizontalOverflow, verticalOverflow);
+    return LevelDom.wrapOverflowEvent(e);
+  }
+
+  bool get horizontalOverflow() => _ptr.horizontalOverflow;
+
+  int get orient() => _ptr.orient;
+
+  bool get verticalOverflow() => _ptr.verticalOverflow;
+}