Add a direct dependency from guava-gwt to checker-qual.
Add a direct dependency from guava-gwt to checker-qual.

The GWT sources (specifically, GwtSerializationDependencies) use @Nullable directly, so we shouldn't rely on relying on it indirectly through guava-jre.

However, what actually prompted this is a strange behavior in Compile-Testing, which Truth uses. This CL should help, though it might not be a fully solution.
https://github.com/google/compile-testing/issues/149

RELNOTES=n/a

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=198882337

EdwardLee03/guava,google/guava,mosoft521/guava,mosoft521/guava,typetools/guava,EdwardLee03/guava,rgoldberg/guava,google/guava,google/guava,typetools/guava,rgoldberg/guava,mosoft521/guava,xasx/guava,xasx/guava