Avoid verifying heap while sweeper is running.

runtime/vm/pages.cc

Index: runtime/vm/pages.cc
===================================================================
--- runtime/vm/pages.cc	(revision 40943)
+++ runtime/vm/pages.cc	(working copy)
@@ -656,7 +656,7 @@
   }
 
   if (FLAG_verify_before_gc) {
-    OS::PrintErr("Verifying before MarkSweep...");
+    OS::PrintErr("Verifying before marking...");
     heap_->Verify();
     OS::PrintErr(" done.\n");
   }
@@ -689,6 +689,11 @@
   int64_t mid3 = 0;
 
   {
+    if (FLAG_verify_before_gc) {
+      OS::PrintErr("Verifying before sweeping...");
+      heap_->Verify(kAllowMarked);
+      OS::PrintErr(" done.\n");
+    }
     GCSweeper sweeper;
 
     // During stop-the-world phases we should use bulk lock when adding elements
@@ -744,6 +749,11 @@
         // Advance to the next page.
         page = next_page;
       }
+      if (FLAG_verify_after_gc) {
+        OS::PrintErr("Verifying after sweeping...");
+        heap_->Verify(kForbidMarked);
+        OS::PrintErr(" done.\n");
+      }
     } else {
       // Start the concurrent sweeper task now.
       GCSweeper::SweepConcurrent(
@@ -772,12 +782,6 @@
     freelist_[HeapPage::kExecutable].Print();
   }
 
-  if (FLAG_verify_after_gc) {
-    OS::PrintErr("Verifying after MarkSweep...");
-    heap_->Verify(FLAG_concurrent_sweep ? kAllowMarked : kForbidMarked);
-    OS::PrintErr(" done.\n");
-  }
-
   // Done, reset the task count.
   {
     MonitorLocker ml(tasks_lock());