Add beforechange event type and provide key with change events

If you know ahead of time that you only want to listen for changes for a specific property, the foo:change type events can be useful.  If you want to listen for changes on all properties, the change event becomes more useful if it provides information on what changed.  And the beforechange event allows listeners to access values before they change.

src/ol/object.js

@@ -10,6 +10,7 @@ goog.provide('ol.ObjectEventType');
 
 goog.require('goog.array');
 goog.require('goog.events');
+goog.require('goog.events.Event');
 goog.require('goog.functions');
 goog.require('goog.object');
 goog.require('ol.Observable');
@@ -19,11 +20,34 @@ goog.require('ol.Observable');
  * @enum {string}
  */
 ol.ObjectEventType = {
+  BEFORECHANGE: 'beforechange',
   CHANGE: 'change'
 };
 
 
 
+/**
+ * Object representing a property change event.
+ *
+ * @param {string} type The event type.
+ * @param {string} key The property name.
+ * @extends {goog.events.Event}
+ * @constructor
+ */
+ol.ObjectEvent = function(type, key) {
+  goog.base(this, type);
+
+  /**
+   * The name of the property whose value is changing.
+   * @type {string}
+   */
+  this.key = key;
+
+};
+goog.inherits(ol.ObjectEvent, goog.events.Event);
+
+
+
 /**
  * @constructor
  * @param {ol.Object} target
@@ -331,7 +355,7 @@ ol.Object.prototype.notify = function(key) {
 ol.Object.prototype.notifyInternal_ = function(key) {
   var eventType = ol.Object.getChangeEventType(key);
   this.dispatchEvent(eventType);
-  this.dispatchEvent(ol.ObjectEventType.CHANGE);
+  this.dispatchEvent(new ol.ObjectEvent(ol.ObjectEventType.CHANGE, key));
 };
 
 
@@ -342,6 +366,7 @@ ol.Object.prototype.notifyInternal_ = function(key) {
  * @todo stability experimental
  */
 ol.Object.prototype.set = function(key, value) {
+  this.dispatchEvent(new ol.ObjectEvent(ol.ObjectEventType.BEFORECHANGE, key));
   var accessors = ol.Object.getAccessors(this);
   if (accessors.hasOwnProperty(key)) {
     var accessor = accessors[key];