Rename String.concat to operator+.

tests/compiler/dart2js_extra/bailout_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.
 
 var myString;
 
 String ifBailout(test) {
   if (test) {
     // Share the same variable for the type inference.
     var o = myString;
@@ skipping 19 matching lines @@
     return '${o[0]} else bailout';
   }
 }
 
 String forBailout() {
   var n = myString.length;
   var res = '';
   for (int i = 0; i < n; i++) {
     var o = myString;
     if (false) o[1] = 2;
-    res = res.concat(o[i]);
+    res += o[i];
   }
   return res;
 }
 
 String forInBailout() {
   var n = myString.length;
   var res = '';
   for (int i in myString.codeUnits) {
     var o = myString;
     if (false) o[1] = 2;
-    res = res.concat(new String.fromCharCodes([i]));
+    res += new String.fromCharCodes([i]);
   }
   return res;
 }
 
 String innerForBailout() {
   var n = myString.length;
   var res = '';
   for (int i = 0; i < 2; i++) {
     for (int j = 0; j < n; j++) {
       var o = myString;
       if (false) o[1] = 2;
-      res = res.concat(o[j]);
+      res += o[j];
     }
   }
   return res;
 }
 
 String whileBailout() {
   var n = myString.length;
   var res = '';
   var i = 0;
   while (i < n) {
     var o = myString;
     if (false) o[1] = 2;
-    res = res.concat(o[i]);
+    res += o[i];
     i++;
   }
   return res;
 }
 
 String doWhileBailout() {
   var n = myString.length;
   var res = '';
   var i = 0;
   do {
     var o = myString;
     if (false) o[1] = 2;
-    res = res.concat(o[i]);
+    res += o[i];
     i++;
   } while (i < n);
   return res;
 }
 
 void phiBailout() {
   var prev = -1;
   bool inside = false;
   for (int i = 0; i < 2; i++) {
     var o = myString;
@@ skipping 110 matching lines @@
   Expect.equals('1234', doWhileBailout());
 
   Expect.equals(102334155, fibonacci(40));
 
   phiBailout();
   ifPhiBailout1(1);
   ifPhiBailout1(2);
   ifPhiBailout2(1);
   ifPhiBailout2(2);
 }