Remove String.concat from implementation classes.

runtime/lib/string_patch.dart

 // Copyright (c) 2012, 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.
 
 patch class String {
   /* patch */ factory String.fromCharCodes(Iterable<int> charCodes) {
     return _StringBase.createFromCharCodes(charCodes);
   }
 }
 
@@ skipping 56 matching lines @@
   int get length native "String_getLength";
 
   bool get isEmpty {
     return this.length == 0;
   }
 
   bool get isNotEmpty => !isEmpty;
 
   String operator +(String other) native "String_concat";
 
-  String concat(String other) => this + other;
-
   String toString() {
     return this;
   }
 
   bool operator ==(Object other) {
     if (identical(this, other)) {
       return true;
     }
     if ((other is !String) ||
         (this.length != other.length)) {
@@ skipping 547 matching lines @@
 class _CodeUnits extends Object with ListMixin<int>,
                                      UnmodifiableListMixin<int> {
   /** The string that this is the code units of. */
   String _string;
 
   _CodeUnits(this._string);
 
   int get length => _string.length;
   int operator[](int i) => _string.codeUnitAt(i);
 }