Eliminate a couple of uses of String +

runtime/vm/custom_isolate_test.cc

 // 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.
 
 #include "include/dart_api.h"
 
 #include "vm/unit_test.h"
 
 // Custom Isolate Test.
 //
@@ skipping 39 matching lines @@
     "\n"
     "interface CustomIsolate default CustomIsolateImpl {\n"
     "  CustomIsolate(String entry);\n"
     "\n"
     "  Future<SendPort> spawn();\n"
     "}\n"
     "\n"
     "isolateMain() {\n"
     "   echo('Running isolateMain');\n"
     "   mainPort.receive((message, SendPort replyTo) {\n"
-    "     echo('Received: ' + message);\n"
+    "     echo('Received: $message');\n"
     "     replyTo.send((message + 1), null);\n"
     "   });\n"
     "}\n"
     "\n"
     "main() {\n"
     "  var isolate = new CustomIsolate(\"isolateMain\");\n"
     "  isolate.spawn().then((SendPort port) {\n"
     "    port.call(42).then((message) {\n"
-    "      echo('Received: ' + message);\n"
+    "      echo('Received: $message');\n"
     "    });\n"
     "  });\n"
     "  return 'success';\n"
     "}\n";
 
 
 // An entry in our event queue.
 class Event {
  protected:
   explicit Event(Dart_Isolate isolate) : isolate_(isolate), next_(NULL) {}
@@ skipping 257 matching lines @@
   OS::Print("-- Finished event loop --\n");
   EXPECT_STREQ("Received: 43", saved_echo);
   free(const_cast<char*>(saved_echo));
 
   delete event_queue;
 }
 
 #endif  // defined(TARGET_ARCH_IA32) || defined(TARGET_ARCH_X64).
 
 }  // namespace dart