Add constructors to all the event classes.

client/html/src/MutationEventWrappingImplementation.dart

Index: client/html/src/MutationEventWrappingImplementation.dart
diff --git a/client/html/src/MutationEventWrappingImplementation.dart b/client/html/src/MutationEventWrappingImplementation.dart
new file mode 100644
index 0000000000000000000000000000000000000000..8986b638d5e3b6a9f8a3e1c5a678cc11d8b6e53c
--- /dev/null
+++ b/client/html/src/MutationEventWrappingImplementation.dart
@@ -0,0 +1,27 @@
+// 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 MutationEventWrappingImplementation extends EventWrappingImplementation implements MutationEvent {
+  MutationEventWrappingImplementation._wrap(ptr) : super._wrap(ptr);
+
+  factory MutationEventWrappingImplementation(String type, Node relatedNode,
+      String prevValue, String newValue, String attrName, int attrChange,
+      [bool canBubble = true, bool cancelable = true]) {
+    final e = dom.document.createEvent("MutationEvent");
+    e.initMutationEvent(type, canBubble, cancelable,
+        LevelDom.unwrap(relatedNode), prevValue, newValue, attrName,
+        attrChange);
+    return LevelDom.wrapMutationEvent(e);
+  }
+
+  int get attrChange() => _ptr.attrChange;
+
+  String get attrName() => _ptr.attrName;
+
+  String get newValue() => _ptr.newValue;
+
+  String get prevValue() => _ptr.prevValue;
+
+  Node get relatedNode() => LevelDom.wrapNode(_ptr.relatedNode);
+}