Make default argument to Iterable.join be "".

runtime/lib/growable_array.dart

Index: runtime/lib/growable_array.dart
diff --git a/runtime/lib/growable_array.dart b/runtime/lib/growable_array.dart
index abfd4f98ca70c2f50a639552f30e72379cffc51b..ba0b90a16db6c22dc5f561cba632a1bef4ed6b1c 100644
--- a/runtime/lib/growable_array.dart
+++ b/runtime/lib/growable_array.dart
@@ -242,11 +242,11 @@ class _GrowableObjectArray<T> implements List<T> {
     }
   }
 
-  String join([String separator]) {
+  String join([String separator = ""]) {
     if (isEmpty) return "";
     if (this.length == 1) return "${this[0]}";
     StringBuffer buffer = new StringBuffer();
-    if (separator == null || separator == "") {
+    if (separator.isEmpty) {
       for (int i = 0; i < this.length; i++) {
         buffer.write("${this[i]}");
       }