is_block_scope return true when current node is an instance of AST_IterationStatement. then the scope of let variable can be figured out accurately (#1561)

lib/scope.js

@@ -331,6 +331,10 @@ AST_Block.DEFMETHOD("is_block_scope", function(){
     );
 });
 
+AST_IterationStatement.DEFMETHOD("is_block_scope", function(){
+     return true;
+});
+
 AST_Lambda.DEFMETHOD("init_scope_vars", function(){
     AST_Scope.prototype.init_scope_vars.apply(this, arguments);
     this.uses_arguments = false;