Import: Add Nonnull to String.valueOf
Import: Add Nonnull to String.valueOf

commit 165f8d82b1278bee6233c47f7deebcb8e5126165
Author: Goktug Gokdogan <goktug@google.com>
Date:   Tue May 23 19:16:27 2017 -0700

    Add Nonnull to String.valueOf

    Otherwise JsCompiler thinks that String.valueOf(a) + 1 might
    result in number.

    Change-Id: I2954d21c279a430b60ab2aebfdf6586180223dd2

PiperOrigin-RevId: 157033911

google/j2cl,google/j2cl,google/j2cl,google/j2cl,google/j2cl