Fixed strong mode errors and warnings reachable from lib/observe.dart

lib/html.dart

 // Copyright (c) 2013, 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.
 
 // TODO(jmesserly): can we handle this more elegantly?
 // In general, it seems like we want a convenient way to take a Stream plus a
 // getter and convert this into an Observable.
 
 /// Helpers for exposing dart:html as observable data.
 library observe.html;
@@ skipping 24 matching lines @@
 
   /// Pushes a new URL state, similar to the affect of clicking a link.
   /// Has no effect if the [value] already equals [window.location.hash].
   @reflectable void set hash(String value) {
     if (value == hash) return;
 
     window.history.pushState(null, '', value);
     _notifyHashChange(null);
   }
 
-  void _notifyHashChange(_) {
+  void _notifyHashChange(Event _) {
     var oldValue = _currentHash;
     _currentHash = hash;
     notifyPropertyChange(#hash, oldValue, _currentHash);
   }
 }
 
 /// *Deprecated* use [CssClassSet.toggle] instead.
 ///
 /// Add or remove CSS class [className] based on the [value].
 @deprecated
 void updateCssClass(Element element, String className, bool value) {
   if (value == true) {
     element.classes.add(className);
   } else {
     element.classes.remove(className);
   }
 }
 
 /// *Deprecated* use `class="{{ binding }}"` in your HTML instead. It will also
 /// work on a `<polymer-element>`.
 ///
 /// Bind a CSS class to the observable [object] and property [path].
 @deprecated
-PathObserver bindCssClass(Element element, String className,
-    Observable object, String path) {
-
+PathObserver bindCssClass(
+    Element element, String className, Observable object, String path) {
   callback(value) {
     updateCssClass(element, className, value);
   }
 
   var obs = new PathObserver(object, path);
   callback(obs.open(callback));
   return obs;
 }