Better error message for Collectors.toMap

Author: zbynek

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

@@ -380,7 +380,7 @@ public static <T> Collector<T,?,List<T>> toList() {
         keyMapper,
         valueMapper,
         (m1, m2) -> {
-          throw new IllegalStateException("Can't assign multiple values to the same key");
+          throw new IllegalStateException("Duplicate key " + m1);
         });
   }
 

user/test/com/google/gwt/emultest/java8/util/stream/CollectorsTest.java

@@ -419,6 +419,7 @@ public void testMap() {
       applyItemsWithoutSplitting(c, "a", "a");
       fail("expected IllegalStateException");
     } catch (IllegalStateException expected) {
+      assertEquals("Duplicate key a", expected.getMessage());
     }
     try {
       applyItemsWithSplitting(c, "a", "a");