Make default argument to Iterable.join be "".

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 e8a6a956ac1fbb62d61f8260cf789b694dc8e042..79fe6ad026a27b5bfdf5520edb554735f17b4d49 100644
--- a/sdk/lib/_collection_dev/iterable.dart
+++ b/sdk/lib/_collection_dev/iterable.dart
@@ -166,9 +166,9 @@ abstract class ListIterable<E> extends Iterable<E> {
     return max;
   }
 
-  String join([String separator]) {
+  String join([String separator = ""]) {
     int length = this.length;
-    if (separator != null && !separator.isEmpty) {
+    if (separator != "") {
       if (length == 0) return "";
       String first = "${elementAt(0)}";
       if (length != this.length) {
@@ -667,7 +667,7 @@ class EmptyIterable<E> extends Iterable<E> {
 
   E max([int compare(E a, E b)]) => null;
 
-  String join([String separator]) => "";
+  String join([String separator = ""]) => "";
 
   Iterable<E> where(bool test(E element)) => this;