Remove workarounds for fixed issue 21620.

runtime/bin/gen_snapshot.cc

 // Copyright (c) 2013, 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.
 
 // Generate a snapshot file after loading all the scripts specified on the
 // command line.
 
 #include <stdlib.h>
 #include <string.h>
 #include <stdio.h>
@@ skipping 1004 matching lines @@
     return 255;
   }
 
   Thread::InitOnce();
   DartUtils::SetOriginalWorkingDirectory();
   // Start event handler.
   TimerUtils::InitOnce();
   EventHandler::Start();
 
   vm_options.AddArgument("--load_deferred_eagerly");
-  // Workaround until issue 21620 is fixed.
-  // (https://github.com/dart-lang/sdk/issues/21620)
-  vm_options.AddArgument("--no-concurrent_sweep");
 
   if (IsSnapshottingForPrecompilation()) {
     vm_options.AddArgument("--precompilation");
 #if TARGET_ARCH_ARM
     // This is for the iPod Touch 5th Generation (and maybe other older devices)
     vm_options.AddArgument("--no-use_integer_division");
 #endif
   }
 
   Dart_SetVMFlags(vm_options.count(), vm_options.arguments());
@@ skipping 113 matching lines @@
   EventHandler::Stop();
   return 0;
 }
 
 }  // namespace bin
 }  // namespace dart
 
 int main(int argc, char** argv) {
   return dart::bin::main(argc, argv);
 }