Linkage Monitor detecting gwt-user problem: googleapis/google-cloud-java#6531.

Linkage Checker invalidates Truth library using gwt-user (provided):

• Google-cloud-firestore 1.27.0 does not use Truth (com.google.truth:truth:1.0).
• Google-cloud-firestore 1.28.1-SNAPSHOT (Ben's migrate-conformance-tests) uses Truth.
• Truth depends on gwt-user (scope: provided)
• gwt-user has shaded classes that have invalid references.

This means Linkage Checker invalidates all projects that use Truth. Can we handle (whitelist) this case (or is this genuine error)?

Why Truth needs gwt-user?

Truth (https://search.maven.org/artifact/com.google.truth/truth/1.0/jar) has a sub-artifact for GWT.

    <!-- Required only to build the -gwt sub-artifact. -->
    <dependency>
      <groupId>com.google.gwt</groupId>
      <artifactId>gwt-user</artifactId>
      <scope>provided</scope>
    </dependency>

I don't see truth-gwt in Maven central: https://search.maven.org/search?q=g:com.google.truth