dart2js implementation of StringBuffer.writeAll that optimizes better

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 e8bc72dcd434feab9d492922d4971555569a3f06..31b87801fd4de6ba386327825ef7d337cf4ced1d 100644
--- a/sdk/lib/core/string_buffer.dart
+++ b/sdk/lib/core/string_buffer.dart
@@ -37,26 +37,9 @@ class StringBuffer implements StringSink {
   /// Adds the string representation of [charCode] to the buffer.
   external void writeCharCode(int charCode);
 
-  void writeAll(Iterable objects, [String separator = ""]) {
-    Iterator iterator = objects.iterator;
-    if (!iterator.moveNext()) return;
-    if (separator.isEmpty) {
-      do {
-        write(iterator.current);
-      } while (iterator.moveNext());
-    } else {
-      write(iterator.current);
-      while (iterator.moveNext()) {
-        write(separator);
-        write(iterator.current);
-      }
-    }
-  }
+  external void writeAll(Iterable objects, [String separator = ""]);
 
-  void writeln([Object obj = ""]) {
-    write(obj);
-    write("\n");
-  }
+  external void writeln([Object obj = ""]);
 
   /**
    * Clears the string buffer.