util.concurrent tests under Guava.

    The problem is that GWT responds poorly to two .gwt.xml files in the same
    Java package; see https://goo.gl/pRV3Yn for details.

    The summary is that it ignores one file in favor of the other.
    util.concurrent, like nearly all our packages, has two .gwt.xml files: one
    for prod and one for tests. However, unlike our other packages, as of this

            <tag>
              <name>spec</name>
              <placement>X</placement>
            </tag>
          </tags>

          <!-- TODO(cpovirk): Move this to the parent after making the package-list files available there. -->
          <!-- We add the link ourselves, both so that we can choose Java 9 over the version that -source suggests and so that we can solve the JSR305 problem described below. -->

          <type>String</type>
        </field>
        <field>
          <name>filePermissions</name>
          <version>1.0.0+</version>
          <description>
            The permissions for files when they are created.
          </description>
          <type>String</type>
        </field>
        <field>
          <name>directoryPermissions</name>
          <version>1.0.0+</version>