Rename String.concat to operator+.

tests/corelib/collection_length_test.dart

 // Copyright (c) 2013, 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.
 
 library map_test;
 import 'dart:collection';
 
 // Test that length/isEmpty opertions are constant time on
 // maps, strings and collections.
 
 void testString(int n) {
   String s = "x";
   String string = "";
   int length = n;
   while (true) {
     if ((length & 1) == 1) {
-      string = string.concat(s);
+      string += s;
     }
     length >>= 1;
     if (length == 0) break;
-    s = s.concat(s);
+    s += s;
   }
   testLength(string, n);
   testLength(string.codeUnits, n);
 }
 
 void testMap(Map map, int n) {
   for (int i = 0; i < n; i++) {
     map[i] = i;
   }
   testLength(map, n);
@@ skipping 34 matching lines @@
   testMap(new LinkedHashMap(), N);
   testMap(new SplayTreeMap(), N);
   testCollection(new HashSet(), N);
   testCollection(new LinkedHashSet(), N);
   testCollection(new ListQueue(), N);
   testList(new List()..length = N, N);
   testList(new List(N), N);
   testString(N);
   // DoubleLinkedQueue has linear length, but fast isEmpty.
 }