Remove workarounds for fixed issue 21620.

runtime/bin/gen_snapshot.cc

Index: runtime/bin/gen_snapshot.cc
diff --git a/runtime/bin/gen_snapshot.cc b/runtime/bin/gen_snapshot.cc
index e15e49c6662b76612b51ee10c78a89cd0d94afec..3e331796d1e128622efc2e63f126064612284eda 100644
--- a/runtime/bin/gen_snapshot.cc
+++ b/runtime/bin/gen_snapshot.cc
@@ -1022,9 +1022,6 @@ int main(int argc, char** argv) {
   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");