Remove workarounds for fixed issue 21620.

runtime/vm/snapshot_test.cc

Index: runtime/vm/snapshot_test.cc
diff --git a/runtime/vm/snapshot_test.cc b/runtime/vm/snapshot_test.cc
index 2b3df5eacc106fe34e2916407ac4886535a4db41..e3da4bf460e4a562771d96891bef32694767edd3 100644
--- a/runtime/vm/snapshot_test.cc
+++ b/runtime/vm/snapshot_test.cc
@@ -1031,10 +1031,6 @@ UNIT_TEST_CASE(CanonicalizationInScriptSnapshots) {
 
   bool saved_load_deferred_eagerly_mode = FLAG_load_deferred_eagerly;
   FLAG_load_deferred_eagerly = true;
-  // Workaround until issue 21620 is fixed.
-  // (https://github.com/dart-lang/sdk/issues/21620)
-  bool saved_concurrent_sweep_mode = FLAG_concurrent_sweep;
-  FLAG_concurrent_sweep = false;
   {
     // Start an Isolate, and create a full snapshot of it.
     TestIsolateScope __test_isolate__;
@@ -1051,7 +1047,6 @@ UNIT_TEST_CASE(CanonicalizationInScriptSnapshots) {
     Dart_ExitScope();
   }
   FLAG_load_deferred_eagerly = saved_load_deferred_eagerly_mode;
-  FLAG_concurrent_sweep = saved_concurrent_sweep_mode;
 
   {
     // Now Create an Isolate using the full snapshot and load the
@@ -1347,10 +1342,6 @@ UNIT_TEST_CASE(ScriptSnapshot) {
 
   bool saved_load_deferred_eagerly_mode = FLAG_load_deferred_eagerly;
   FLAG_load_deferred_eagerly = true;
-  // Workaround until issue 21620 is fixed.
-  // (https://github.com/dart-lang/sdk/issues/21620)
-  bool saved_concurrent_sweep_mode = FLAG_concurrent_sweep;
-  FLAG_concurrent_sweep = false;
   {
     // Start an Isolate, and create a full snapshot of it.
     TestIsolateScope __test_isolate__;
@@ -1367,7 +1358,6 @@ UNIT_TEST_CASE(ScriptSnapshot) {
     Dart_ExitScope();
   }
   FLAG_load_deferred_eagerly = saved_load_deferred_eagerly_mode;
-  FLAG_concurrent_sweep = saved_concurrent_sweep_mode;
 
   // Test for Dart_CreateScriptSnapshot.
   {