Make default argument to Iterable.join be "".

runtime/lib/array.dart

Index: runtime/lib/array.dart
diff --git a/runtime/lib/array.dart b/runtime/lib/array.dart
index aad4e38126f37e07938699b4dc2b071c5bec343f..6899b2840c00ef3925354c031a6520dfe8cd27e7 100644
--- a/runtime/lib/array.dart
+++ b/runtime/lib/array.dart
@@ -106,7 +106,7 @@ class _ObjectArray<E> implements List<E> {
     IterableMixinWorkaround.forEach(this, f);
   }
 
-  String join([String separator]) {
+  String join([String separator = ""]) {
     return IterableMixinWorkaround.joinList(this, separator);
   }
 
@@ -357,7 +357,7 @@ class _ImmutableArray<E> implements List<E> {
     return IterableMixinWorkaround.mapList(this, f);
   }
 
-  String join([String separator]) {
+  String join([String separator = ""]) {
     return IterableMixinWorkaround.joinList(this, separator);
   }