Fix renaming of exception variables on left-hand-sides.

pkg/js_ast/lib/src/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 593 matching lines @@
   int get precedenceLevel => EXPRESSION;
 }
 
 class Assignment extends Expression {
   final Expression leftHandSide;
   final String op;         // Null, if the assignment is not compound.
   final Expression value;  // May be null, for [VariableInitialization]s.
 
   Assignment(leftHandSide, value)
       : this.compound(leftHandSide, null, value);
+  // If `this.op == null` this will be a non-compound assignment.
   Assignment.compound(this.leftHandSide, this.op, this.value);
 
   int get precedenceLevel => ASSIGNMENT;
 
   bool get isCompound => op != null;
 
   accept(NodeVisitor visitor) => visitor.visitAssignment(this);
 
   void visitChildren(NodeVisitor visitor) {
     leftHandSide.accept(visitor);
@@ skipping 548 matching lines @@
 class Comment extends Statement {
   final String comment;
 
   Comment(this.comment);
 
   accept(NodeVisitor visitor) => visitor.visitComment(this);
   Comment _clone() => new Comment(comment);
 
   void visitChildren(NodeVisitor visitor) {}
 }