Make check() final.

No one overrides it, and no one should.

RELNOTES=Made `StandardSubjectBuilder.check()` final.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=179961554

core/src/main/java/com/google/common/truth/Subject.java

@@ -339,8 +339,7 @@ protected String actualCustomStringRepresentation() {
    * ThrowableSubject#hasMessageThat} is implemented with {@code
    * check().that(actual().getMessage()}, which returns a {@link StringSubject}.
    */
-  // TODO(diamondm) this should be final, can we do that safely?
-  protected StandardSubjectBuilder check() {
+  protected final StandardSubjectBuilder check() {
     return new StandardSubjectBuilder(metadata);
   }