Introduce a flag to disable string operator +

runtime/lib/string.dart

Index: runtime/lib/string.dart
===================================================================
--- runtime/lib/string.dart	(revision 6383)
+++ runtime/lib/string.dart	(working copy)
@@ -233,11 +233,17 @@
   }
 
   /**
+   * TODO(hausner): remove support for operator + and native method plus().
+   */
+  String plus(String other) native "String_plus";
+
+  /**
    * Convert argument obj to string and concat it with this string.
    * Returns concatenated string.
+   * TODO(hausner): remove support for +.
    */
   String operator +(Object obj) {
-    return this.concat(obj.toString());
+    return this.plus(obj.toString());
   }
 
   /**