Add optional writeAll separator argument.

sdk/lib/io/io_sink.dart

Index: sdk/lib/io/io_sink.dart
diff --git a/sdk/lib/io/io_sink.dart b/sdk/lib/io/io_sink.dart
index 7c564fae6b7f5df6a76d6c53330f2c1fdad8533c..71426da8f678ad427ff403e791961412241dfe97 100644
--- a/sdk/lib/io/io_sink.dart
+++ b/sdk/lib/io/io_sink.dart
@@ -94,8 +94,20 @@ class _IOSinkImpl<T> implements IOSink<T> {
     writeBytes(_encodeString(string, _encoding));
   }
 
-  void writeAll(Iterable objects) {
-    for (Object obj in objects) write(obj);
+  void writeAll(Iterable objects, [String separator = ""]) {
+    Iterator iterator = objects.iterator;
+    if (!iterator.moveNext()) return;
+    if (separator == "") {
+      do {
+        write(iterator.current);
+      } while (iterator.moveNext());
+    } else {
+      write(iterator.current);
+      while (iterator.moveNext()) {
+        write(separator);
+        write(iterator.current);
+      }
+    }
   }
 
   void writeln([Object obj = ""]) {