Each mapped stream must be closed after its contents have been placed… (#9949)

… downstream

Author: treblereel

user/super/com/google/gwt/emul/java/util/stream/Collectors.java

@@ -177,13 +177,14 @@ public static Collector<CharSequence,?,String> joining(CharSequence delimiter) {
     return new CollectorImpl<>(
         downstream.supplier(),
         (A a, T t) -> {
-            Stream<? extends U> stream = mapper.apply(t);
+          try (Stream<? extends U> stream = mapper.apply(t)) {
             if (stream == null) {
               return;
             }
             stream.forEach(u -> {
               downstream.accumulator().accept(a, u);
             });
+          }
         },
         downstream.combiner(),
         downstream.finisher()

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

@@ -49,6 +49,11 @@ public void testFlatMapping() {
       return items.stream();
     }, toList());
     applyItems(Arrays.asList("a"), flatMappingToNull, Arrays.asList("a"), Arrays.asList("b", "c"));
+
+    int[] calledCount = {0};
+    Stream<String> mapped = Stream.of("x").onClose(() -> calledCount[0]++);
+    Stream.of(1).collect(flatMapping(x -> mapped, toList()));
+    assertEquals(1, calledCount[0]);
   }
 
   public void testFiltering() {