don't hoist regenerator variable declarations in system module formatter

lib/6to5/transformation/modules/system.js

@@ -111,16 +111,20 @@ var hoistVariablesVisitor = {
         return;
       }
 
+      // ignore block hoisted nodes as these can be left in
+      if (node._blockHoist) return;
+
       var nodes = [];
 
-      _.each(node.declarations, function (declar) {
+      for (var i = 0; i < node.declarations.length; i++) {
+        var declar = node.declarations[i];
         hoistDeclarators.push(t.variableDeclarator(declar.id));
         if (declar.init) {
           // no initializer so we can just hoist it as-is
           var assign = t.expressionStatement(t.assignmentExpression("=", declar.id, declar.init));
           nodes.push(assign);
         }
-      });
+      }
 
       // for (var i in test)
       // for (var i = 0;;)