Rename String.concat to operator+.

runtime/tests/vm/dart/inline_stack_frame_test.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.
 
 // This test tries to verify that we produce the correct stack trace when
 // throwing exceptions even when functions are inlined.
 // The test invokes a bunch of functions and then does a throw. There is a
 // catch at the outer function which uses the stack trace produced to return
 // a string. The test then verifies that the stack trace contains each
 // method in the invocation chain. The test is run during warmup to ensure
 // unoptimized code produces the correct result and is then run
 // in a loop to ensure optimization kicks in and some inlining is done.
 // Note: it appears that functions which have a throw are not inlined (func4)
 //       and functions that have try/catch in them are not optimized (func1).
 //       func2 is not inlined as func1 has not been optimized.
 
 class Test {
   String func1(var k) {
     try {
       for (var i = 0; i <= 50; i++) {
         func2(i * k);
       }
       return "";
     } catch (e, stacktrace) {
-      var result = e.concat(stacktrace.toString());
+      var result = e + stacktrace.toString();
       return result;
     }
   }
   int func2(var i) {
     var result = 0;
     for (var k = 0; k <= 10; k++) {
       result += func3(i + k);
     }
     return result;
   }
@@ skipping 29 matching lines @@
   for (var i = 0; i <= 200; i++) {
     result = x.func1(i);
   }
   Expect.isTrue(result.contains("show me inlined functions"));
   Expect.isTrue(result.contains("Test.func1"));
   Expect.isTrue(result.contains("Test.func2"));
   Expect.isTrue(result.contains("Test.func3"));
   Expect.isTrue(result.contains("Test.func4"));
   Expect.isTrue(result.contains("Test.func5"));
 }