Rename String.concat to operator+.

tests/corelib/string_test.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.
 
 // TODO(ngeoffray): test String methods with null arguments.
 class StringTest {
 
   static testMain() {
     testOutOfRange();
     testIllegalArgument();
@@ skipping 47 matching lines @@
   static testCodeUnitAt() {
     String str = "string";
     for (int i = 0; i < str.length; i++) {
       Expect.equals(true, str.codeUnitAt(i) is int);
     }
   }
 
   static testConcat() {
     var a = "One";
     var b = "Four";
-    var c = a.concat(b);
+    var c = a + b;
     Expect.equals(7, c.length);
     Expect.equals("OneFour", c);
   }
 
   static testEquals() {
     Expect.equals("str", "str");
 
-    Expect.equals("str", "s".concat("t").concat("r"));
-    Expect.equals("s".concat("t").concat("r"), "str");
+    Expect.equals("str", "s" + "t" + "r");
+    Expect.equals("s" + "t" + "r", "str");
 
     Expect.equals(false, "str" == "s");
     Expect.equals(false, "str" == "r");
     Expect.equals(false, "str" == "st");
     Expect.equals(false, "str" == "tr");
 
     Expect.equals(false, "s" == "str");
     Expect.equals(false, "r" == "str");
     Expect.equals(false, "st" == "str");
     Expect.equals(false, "tr" == "str");
@@ skipping 192 matching lines @@
     }
     test("abc");
     test("");
     test(" ");
   }
 }
 
 main() {
   StringTest.testMain();
 }