Implement try/catch without finally, and without type checks.

frog/leg/frog_leg.dart

 // Copyright (c) 2012, 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.
 
 #library('frog_leg');
 
 #import('../../lib/uri/uri.dart');
 #import('../lang.dart', prefix: 'frog');
 #import('elements/elements.dart');
 #import('io/io.dart', prefix: 'io');
@@ skipping 89 matching lines @@
     Script script = currentScript();
     if (node !== null) {
       print(spanFromNode(node, script).toMessageString("cancel leg: $reason"));
     } else if (token !== null) {
       String tokenString = token.toString();
       int begin = token.charOffset;
       int end = begin + tokenString.length;
       print(script.file.getLocationMessage("cancel leg: $reason",
                                            begin, end, true));
     } else if (element !== null) {
-      currentElement = element;
-      cancel(reason: reason, token: element.position());
+      withCurrentElement(element,
+          () => cancel(reason: reason, token: element.position()));
     }
     if (throwOnError) {
       throw new AbortLeg(reason);
     }
     super.cancel(reason, node, token, instruction);
   }
 }
 
 class AbortLeg {
   final message;
   AbortLeg(this.message);
   toString() => 'Aborted due to --throw-on-error: $message';
 }