Add missing import, fix errorprone error (#9950)

Follow-up #9949

Author: niloc132

user/test/com/google/gwt/emultest/java9/util/stream/CollectorsTest.java

@@ -27,6 +27,7 @@
 import java.util.Collections;
 import java.util.List;
 import java.util.stream.Collector;
+import java.util.stream.Stream;
 
 /**
  * Tests for java.util.stream.Collectors Java 9 API emulation.
@@ -49,7 +50,10 @@ public void testFlatMapping() {
       return items.stream();
     }, toList());
     applyItems(Arrays.asList("a"), flatMappingToNull, Arrays.asList("a"), Arrays.asList("b", "c"));
+  }
 
+  @SuppressWarnings("ReturnValueIgnored")
+  public void testFlatMappingClose() {
     int[] calledCount = {0};
     Stream<String> mapped = Stream.of("x").onClose(() -> calledCount[0]++);
     Stream.of(1).collect(flatMapping(x -> mapped, toList()));