Make default argument to Iterable.join be "".

sdk/lib/collection/list.dart

Index: sdk/lib/collection/list.dart
diff --git a/sdk/lib/collection/list.dart b/sdk/lib/collection/list.dart
index 8c9dcd086dde71344cc7b55f95ec5f2ba805883e..a99cf1112642113e5b2ef6e4bd009245128a8913 100644
--- a/sdk/lib/collection/list.dart
+++ b/sdk/lib/collection/list.dart
@@ -181,9 +181,9 @@ abstract class ListMixin<E> implements List<E> {
     return max;
   }
 
-  String join([String separator]) {
+  String join([String separator = ""]) {
     int length = this.length;
-    if (separator != null && !separator.isEmpty) {
+    if (separator != "") {

[sra1, 2013/04/10 09:03:22]

['x','y'].join(null)  ->  "xnully"

is this intentional?  It means any caller forwarding an optional parameter needs
to do explicit defaulting.

[floitsch, 2013/04/10 09:20:37]

Changed. Now it's using "isEmpty" again. You still need to do explicit
defaulting. And that's intentional, but if you pass in 'null' it will throw now.
I actually had to change a test-case because it was triggering this behavior.

       if (length == 0) return "";
       String first = "${this[0]}";
       if (length != this.length) {