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 257 matching lines @@
   accept(NodeVisitor visitor) => visitor.visitEmptyStatement(this);
   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(this.condition, this.then, Node otherwise) :

[Jacob, 2015/03/27 17:04:30]

We could alternately make this constructor assert if otherwise is null but I
think that will just make upstream code less convenient to write.

[Jennifer Messerly, 2015/03/27 17:15:54]

Agreed. Another option would be to just have null `otherwise` and change
Printer, as most other AST nodes here use `null` to mean missing. But this looks
good too.

+      otherwise = otherwise != null ? otherwise : new EmptyStatement();
   If.noElse(this.condition, this.then) : this.otherwise = new EmptyStatement();
 
   bool get hasElse => otherwise is !EmptyStatement;
 
   accept(NodeVisitor visitor) => visitor.visitIf(this);
 
   void visitChildren(NodeVisitor visitor) {
     condition.accept(visitor);
     then.accept(visitor);
     otherwise.accept(visitor);
@@ skipping 1051 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);
 }