Fix List.map.join with no separator.

tests/corelib/iterable_join_test.dart

Index: tests/corelib/iterable_join_test.dart
diff --git a/tests/corelib/iterable_join_test.dart b/tests/corelib/iterable_join_test.dart
index 707abf1716fc8d53e90f5f821469363bbd045caf..b09227fa96a92c4fc1ef88e38a0cc9007e7214af 100644
--- a/tests/corelib/iterable_join_test.dart
+++ b/tests/corelib/iterable_join_test.dart
@@ -55,6 +55,13 @@ testCollections() {
   }
   testArray(fixedArray);
   testArray(const [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
+
+  testJoin("a,b,c,d", ["a", "b", "c", "d"].map((x) => x), ",");
+  testJoin("abcd", ["a", "b", "c", "d"].map((x) => x), "");
+  testJoin("abcd", ["a", "b", "c", "d"].map((x) => x));
+  testJoin("null,b,c,d", [null,"b","c","d"].map((x) => x), ",");
+  testJoin("1,2,3,4", [1, 2, 3, 4].map((x) => x), ",");
+  testJoin("4,5,6,7", [ic, ic, ic, ic].map((x) => x), ",");
 }
 
 main() {