Add optional writeAll separator argument.

sdk/lib/core/iterable.dart

Index: sdk/lib/core/iterable.dart
diff --git a/sdk/lib/core/iterable.dart b/sdk/lib/core/iterable.dart
index 604c4a88940ff9bbe8cf1dc08cd88ded2e160531..83618ce3c2f3936a9c59450dad520476f0829182 100644
--- a/sdk/lib/core/iterable.dart
+++ b/sdk/lib/core/iterable.dart
@@ -128,20 +128,8 @@ abstract class Iterable<E> {
    * string.
    */
   String join([String separator]) {
-    Iterator<E> iterator = this.iterator;
-    if (!iterator.moveNext()) return "";
     StringBuffer buffer = new StringBuffer();
-    if (separator == null || separator == "") {
-      do {
-        buffer.write("${iterator.current}");
-      } while (iterator.moveNext());
-    } else {
-      buffer.write("${iterator.current}");
-      while (iterator.moveNext()) {
-        buffer.write(separator);
-        buffer.write("${iterator.current}");
-      }
-    }
+    buffer.writeAll(this, separator == null ? "" : separator);
     return buffer.toString();
   }