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diff --git a/dom/svg/test/MutationEventChecker.js b/dom/svg/test/MutationEventChecker.js
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+++ b/dom/svg/test/MutationEventChecker.js
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+// Helper class to check DOM MutationEvents
+//
+// Usage:
+//
+// * Create a new event checker:
+//     var eventChecker = new MutationEventChecker;
+// * Set the attribute to watch
+//     eventChecker.watchAttr(<DOM element>, "<attribute name>");
+// * Set the events to expect (0..n)
+//     eventChecker.expect("add", "modify");
+//     OR
+//     eventChecker.expect("add modify");
+//     OR
+//     eventChecker.expect(MutationEvent.ADDITION, MutationEvent.MODIFICATION);
+//
+//  An empty string or empty set of arguments is also fine as a way of checking
+//  that all expected events have been received and indicating no events are
+//  expected from the following code, e.g.
+//
+//    eventChecker.expect("");
+//    // changes that are not expected to generate events
+//    eventChecker.expect("modify");
+//    // change that is expected to generate an event
+//    ...
+//
+// * Either finish listening or set the next attribute to watch
+//     eventChecker.finish();
+//     eventChecker.watchAttr(element, "nextAttribute");
+//
+//   In either case a check is performed that all expected events have been
+//   received.
+//
+// * Event checking can be temporarily disabled with ignoreEvents(). The next
+//   call to expect() will cause it to resume.
+
+function MutationEventChecker()
+{
+  this.expectedEvents = [];
+
+  this.watchAttr = function(element, attr)
+  {
+    if (this.attr) {
+      this.finish();
+    }
+
+    this.expectedEvents = [];
+    this.element        = element;
+    this.attr           = attr;
+    this.oldValue       = element.getAttribute(attr);
+    this.giveUp         = false;
+    this.ignore         = false;
+
+    this.element.addEventListener('DOMAttrModified', this._listener, false);
+  }
+
+  this.expect = function()
+  {
+    if (this.giveUp) {
+      return;
+    }
+
+    ok(this.expectedEvents.length == 0,
+       "Expecting new events for " + this.attr +
+       " but the following previously expected events have still not been " +
+       "received: " + this._stillExpecting());
+    if (this.expectedEvents.length != 0) {
+      this.giveUp = true;
+      return;
+    }
+
+    this.ignore = false;
+
+    if (arguments.length == 0 ||
+        arguments.length == 1 && arguments[0] == "") {
+      return;
+    }
+
+    // Turn arguments object into an array
+    var args = Array.prototype.slice.call(arguments);
+    // Check for whitespace separated keywords
+    if (args.length == 1 && typeof args[0] === 'string' &&
+        args[0].indexOf(' ') > 0) {
+      args = args[0].split(' ');
+    }
+    // Convert strings to event Ids
+    this.expectedEvents = args.map(this._argToEventId);
+  }
+
+  // Temporarily disable event checking
+  this.ignoreEvents = function()
+  {
+    // Check all events have been received
+    ok(this.giveUp || this.expectedEvents.length == 0,
+      "Going to ignore subsequent events on " + this.attr +
+      " attribute, but we're still expecting the following events: " +
+      this._stillExpecting());
+
+    this.ignore = true;
+  }
+
+  this.finish = function()
+  {
+    // Check all events have been received
+    ok(this.giveUp || this.expectedEvents.length == 0,
+      "Finishing listening to " + this.attr +
+      " attribute, but we're still expecting the following events: " +
+      this._stillExpecting());
+
+    this.element.removeEventListener('DOMAttrModified', this._listener, false);
+    this.attr = "";
+  }
+
+  this._receiveEvent = function(e)
+  {
+    if (this.giveUp || this.ignore) {
+      this.oldValue = e.newValue;
+      return;
+    }
+
+    // Make sure we're expecting something at all
+    if (this.expectedEvents.length == 0) {
+      ok(false, 'Unexpected ' + this._eventToName(e.attrChange) +
+         ' event when none expected on ' + this.attr + ' attribute.');
+      return;
+    }
+
+    var expectedEvent = this.expectedEvents.shift();
+
+    // Make sure we got the event we expected
+    if (e.attrChange != expectedEvent) {
+      ok(false, 'Unexpected ' + this._eventToName(e.attrChange) +
+        ' on ' + this.attr + ' attribute. Expected ' +
+        this._eventToName(expectedEvent) + ' (followed by: ' +
+        this._stillExpecting() + ")");
+      // If we get events out of sequence, it doesn't make sense to do any
+      // further testing since we don't really know what to expect
+      this.giveUp = true;
+      return;
+    }
+
+    // Common param checking
+    is(e.target, this.element,
+       'Unexpected node for mutation event on ' + this.attr + ' attribute');
+    is(e.attrName, this.attr, 'Unexpected attribute name for mutation event');
+
+    // Don't bother testing e.relatedNode since Attr nodes are on the way
+    // out anyway (but then, so are mutation events...)
+
+    // Event-specific checking
+    if (e.attrChange == MutationEvent.MODIFICATION) {
+      ok(this.element.hasAttribute(this.attr),
+         'Attribute not set after modification');
+      is(e.prevValue, this.oldValue,
+         'Unexpected old value for modification to ' + this.attr +
+         ' attribute');
+      isnot(e.newValue, this.oldValue,
+         'Unexpected new value for modification to ' + this.attr +
+         ' attribute');
+    } else if (e.attrChange == MutationEvent.REMOVAL) {
+      ok(!this.element.hasAttribute(this.attr), 'Attribute set after removal');
+      is(e.prevValue, this.oldValue,
+         'Unexpected old value for removal of ' + this.attr +
+         ' attribute');
+      // DOM 3 Events doesn't say what value newValue will be for a removal
+      // event but generally empty strings are used for other events when an
+      // attribute isn't relevant
+      ok(e.newValue === "",
+         'Unexpected new value for removal of ' + this.attr +
+         ' attribute');
+    } else if (e.attrChange == MutationEvent.ADDITION) {
+      ok(this.element.hasAttribute(this.attr),
+         'Attribute not set after addition');
+      // DOM 3 Events doesn't say what value prevValue will be for an addition
+      // event but generally empty strings are used for other events when an
+      // attribute isn't relevant
+      ok(e.prevValue === "",
+         'Unexpected old value for addition of ' + this.attr +
+         ' attribute');
+      ok(typeof(e.newValue) == 'string' && e.newValue !== "",
+         'Unexpected new value for addition of ' + this.attr +
+         ' attribute');
+    } else {
+      ok(false, 'Unexpected mutation event type: ' + e.attrChange);
+      this.giveUp = true;
+    }
+    this.oldValue = e.newValue;
+  }
+  this._listener = this._receiveEvent.bind(this);
+
+  this._stillExpecting = function()
+  {
+    if (this.expectedEvents.length == 0) {
+      return "(nothing)";
+    }
+    var eventNames = [];
+    for (var i=0; i < this.expectedEvents.length; i++) {
+      eventNames.push(this._eventToName(this.expectedEvents[i]));
+    }
+    return eventNames.join(", ");
+  }
+
+  this._eventToName = function(evtId)
+  {
+    switch (evtId)
+    {
+    case MutationEvent.MODIFICATION:
+      return "modification";
+    case MutationEvent.ADDITION:
+      return "addition";
+    case MutationEvent.REMOVAL:
+      return "removal";
+    }
+  }
+
+  this._argToEventId = function(arg)
+  {
+    if (typeof arg === 'number')
+      return arg;
+
+    if (typeof arg !== 'string') {
+      ok(false, "Unexpected event type: " + arg);
+      return 0;
+    }
+
+    switch (arg.toLowerCase())
+    {
+    case "mod":
+    case "modify":
+    case "modification":
+      return MutationEvent.MODIFICATION;
+
+    case "add":
+    case "addition":
+      return MutationEvent.ADDITION;
+
+    case "removal":
+    case "remove":
+      return MutationEvent.REMOVAL;
+
+    default:
+      ok(false, "Unexpected event name: " + arg);
+      return 0;
+    }
+  }
+}