Fix try catch crasher bug due to null else case.

lib/src/js/nodes.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.
 
 part of js_ast;
 
 abstract class NodeVisitor<T> {
   T visitProgram(Program node);
 
   T visitBlock(Block node);
@@ skipping 258 matching lines @@
   void visitChildren(NodeVisitor visitor) {}
   EmptyStatement _clone() => new EmptyStatement();
 }
 
 class If extends Statement {
   final Expression condition;
   final Node then;
   final Node otherwise;
 
   If(this.condition, this.then, this.otherwise);
-  If.noElse(this.condition, this.then) : this.otherwise = new EmptyStatement();
+  If.noElse(this.condition, this.then) : this.otherwise = null;
 
-  bool get hasElse => otherwise is !EmptyStatement;
+  bool get hasElse => otherwise != null;
 
   accept(NodeVisitor visitor) => visitor.visitIf(this);
 
   void visitChildren(NodeVisitor visitor) {
     condition.accept(visitor);
     then.accept(visitor);
-    otherwise.accept(visitor);
+    if (otherwise != null) otherwise.accept(visitor);
   }
 
   If _clone() => new If(condition, then, otherwise);
 }
 
 abstract class Loop extends Statement {
   final Statement body;
   Loop(this.body);
 }
 
@@ skipping 1041 matching lines @@
   final Expression expression;
 
   CommentExpression(this.comment, this.expression);
 
   int get precedenceLevel => PRIMARY;
   accept(NodeVisitor visitor) => visitor.visitCommentExpression(this);
   CommentExpression _clone() => new CommentExpression(comment, expression);
 
   void visitChildren(NodeVisitor visitor) => expression.accept(visitor);
 }