Add optional writeAll separator argument.

sdk/lib/core/string_buffer.dart

Index: sdk/lib/core/string_buffer.dart
diff --git a/sdk/lib/core/string_buffer.dart b/sdk/lib/core/string_buffer.dart
index 50bb51c4249c67031d4d2ce6bb674eac1a8d1c2c..6ecdc571d985d6a4bb5ff192213c99a96b0112f5 100644
--- a/sdk/lib/core/string_buffer.dart
+++ b/sdk/lib/core/string_buffer.dart
@@ -29,8 +29,20 @@ class StringBuffer implements StringSink {
   /// Adds the string representation of [charCode] to the buffer.
   external void writeCharCode(int charCode);
 
-  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 = ""]) {