Beef up timer cancellation tests

testing/resources/bug_620428.in

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 function fireTimeOut() {
   app.alert("hello world");
 }
 function fireInterval() {
   app.alert("goodbye world");
 }
 var timer = app.setTimeOut("fireTimeOut()", 3000);
 var interval = app.setInterval("fireInterval()", 1000);
+// Clear timers before they fire.
 app.clearTimeOut(timer);
 app.clearInterval(interval);
+// Check that clearing a cleared timer is ok.
+app.clearTimeOut(timer);
+app.clearInterval(interval);
+// Check that invoking the wrong clear method is ok
+var timer = app.setTimeOut("fireTimeOut()", 3000);
+var interval = app.setInterval("fireInterval()", 1000);
+app.clearTimeOut(interval);
+app.clearInterval(timer);
+// Check that clearing a non-timer is ok.
+app.clearTimeOut({"size": 42});
+app.clearInterval({"size": 42});
+// Be sure all of this code ran to completion.
+app.alert("done");
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