Eliminate a couple of uses of String +

runtime/vm/snapshot_test.cc

 // 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.
 
 #include "include/dart_debugger_api.h"
 #include "platform/assert.h"
 #include "vm/bigint_operations.h"
 #include "vm/class_finalizer.h"
 #include "vm/dart_api_impl.h"
 #include "vm/dart_api_message.h"
@@ skipping 1441 matching lines @@
   // Create a native port for posting from C to Dart
   TestIsolateScope __test_isolate__;
   const char* kScriptChars =
       "#import('dart:isolate');\n"
       "void main() {\n"
       "  var messageCount = 0;\n"
       "  var exception = '';\n"
       "  var port = new ReceivePort();\n"
       "  port.receive((message, replyTo) {\n"
       "    if (messageCount < 7) {\n"
-      "      exception += message.toString();\n"
+      "      exception = '$exception${message}';\n"
       "    } else {\n"
-      "      exception += message.length;\n"
+      "      exception = '$exception${message.length}';\n"
       "      for (int i = 0; i < message.length; i++) {\n"
-      "        exception += message[i];\n"
+      "        exception = '$exception${message[i]}';\n"
       "      }\n"
       "    }\n"
       "    messageCount++;\n"
       "    if (messageCount == 8) throw new Exception(exception);\n"
       "  });\n"
       "  return port.toSendPort();\n"
       "}\n";
   Dart_Handle lib = TestCase::LoadTestScript(kScriptChars, NULL);
   Dart_EnterScope();
 
@@ skipping 61 matching lines @@
   EXPECT(Dart_ErrorHasException(result));
   EXPECT_SUBSTRING("Exception: nulltruefalse1234563.14[]100123456789\n",
                    Dart_GetError(result));
 
   Dart_ExitScope();
 }
 
 #endif  // defined(TARGET_ARCH_IA32) || defined(TARGET_ARCH_X64).
 
 }  // namespace dart