Introduce a flag to disable string operator +

runtime/lib/string.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * [StringBase] contains common methods used by concrete String implementations,
  * e.g., OneByteString.
  */
 class StringBase {
 
@@ skipping 215 matching lines @@
     StringBuffer buffer = new StringBuffer();
     int startIndex = 0;
     for (Match match in pattern.allMatches(this)) {
       buffer.add(this.substring(startIndex, match.start())).add(replacement);
       startIndex = match.end();
     }
     return buffer.add(this.substring(startIndex)).toString();
   }
 
   /**
+   * 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());
   }
 
   /**
    * Convert all objects in [values] to strings and concat them
    * into a result string.
    */
   static String _interpolate(List values) {
     int numValues = values.length;
     List<String> stringList = new List<String>(numValues);
     int resultLength = 0;
@@ skipping 254 matching lines @@
     for (int g in groups) {
       result.add(group(g));
     }
     return result;
   }
 
   final int _start;
   final String str;
   final String pattern;
 }