Fix List.map.join with no separator.

sdk/lib/_collection_dev/iterable.dart

Index: sdk/lib/_collection_dev/iterable.dart
diff --git a/sdk/lib/_collection_dev/iterable.dart b/sdk/lib/_collection_dev/iterable.dart
index 89b9e6d4aab3f32e32f475686353a3773d55d1b6..7510273a20e9720c13f08b1220589447b98d7ba0 100644
--- a/sdk/lib/_collection_dev/iterable.dart
+++ b/sdk/lib/_collection_dev/iterable.dart
@@ -279,7 +279,9 @@ class MappedListIterable<S, T> extends Iterable<T> {
       throw new ConcurrentModificationError(_list);
     }
     for (int i = start + 1; i < end; i++) {
-      buffer.add(separator);
+      if (separator != null && separator != "") {
+        buffer.add(separator);
+      }
       buffer.add("${_f(_list[i])}");
       if (_list.length != length) {
         throw new ConcurrentModificationError(_list);