=str,htp clean up build warnings

- explicitly provide Unit values and place parens around tuple creation
- remove structural type usage in TestUtils
- fix Java double-casts
- use unused Java values by asserting their non-nullness
- work around inability to place case class in trait (scripted test)

The remaining warnings about using private types in public methods are
bogus as reported in https://issues.scala-lang.org/browse/SI-9490.

akka-stream-tests/src/test/java/akka/stream/javadsl/SourceTest.java

@@ -566,7 +566,7 @@ public class SourceTest extends StreamTest {
     final Source<Integer, ?> source1 = Source.from(Arrays.asList(0, 1));
     final Source<Integer, ?> source2 = Source.from(Arrays.asList(2, 3));
 
-    final Source<Integer, ?> source = Source.combine(source1, source2, new ArrayList(),
+    final Source<Integer, ?> source = Source.combine(source1, source2, new ArrayList<Source<Integer, ?>>(),
             new Function<Integer, Graph<UniformFanInShape<Integer, Integer>, BoxedUnit>>() {
               public Graph<UniformFanInShape<Integer, Integer>, BoxedUnit> apply(Integer elem) {
                 return Merge.create(elem);