Class ‘WC_Session’ not found in class-wc-session-handler.php

I have plugin "Woocommerce Email Cart" and it was running fine on Woocommerce 2.2. When I upgrade that to Woocommerce 2.5.2 I got this few errors including this. I fixed the other issues but I’m unable to fix this.
I got error saying:-

Fatal error: Class ‘WC_Session’ not found in
/home/31776xa/public_html/wp-content/plugins/woocommerce/includes/class-wc-session-handler.php
on line 18

$woocommerce->frontend_includes();
$session_class = apply_filters( 'woocommerce_session_handler', 'WC_Session_Handler' );
$woocommerce->session  = new $session_class();
$woocommerce->cart     = new WC_Cart();  
$woocommerce->customer = new WC_Customer();

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