Fix Collectors.groupingBy javac error

Change-Id: I99595c1afae47a23ae510d2c9c41b83f6afc8a2d

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

diff --git a/user/super/com/google/gwt/emul/java/util/stream/Collectors.java b/user/super/com/google/gwt/emul/java/util/stream/Collectors.java
index 751f231..cb67745 100644
--- a/user/super/com/google/gwt/emul/java/util/stream/Collectors.java
+++ b/user/super/com/google/gwt/emul/java/util/stream/Collectors.java
@@ -93,7 +93,11 @@
       Function<? super T, ? extends K> classifier,
       Supplier<M> mapFactory,
       Collector<? super T, A, D> downstream) {
-    return groupingBy0(LinkedHashMap::new, classifier, mapFactory, downstream);
+    return groupingBy0(() -> {
+      // cannot use LinkedHashMap::new because javac cannot infer correct
+      // return type of method reference
+      return new LinkedHashMap<>();
+    }, classifier, mapFactory, downstream);
   }
 
   private static <T, K, D, A, M extends Map<K, D>> Collector<T, ?, M> groupingBy0(