Implement a 2-pass heap verification algorithm.

runtime/vm/scavenger.cc

Index: runtime/vm/scavenger.cc
diff --git a/runtime/vm/scavenger.cc b/runtime/vm/scavenger.cc
index 2bf47b7864c6932d7706d72afe7265b616172817..3102d317f33b3e498e36a58641a47493f3c051f7 100644
--- a/runtime/vm/scavenger.cc
+++ b/runtime/vm/scavenger.cc
@@ -380,7 +380,7 @@ void Scavenger::Scavenge(bool invoke_api_callbacks) {
   NoHandleScope no_handles(isolate);
 
   if (FLAG_verify_before_gc) {
-    OS::PrintErr("Verifying before Scavenge... ");
+    OS::PrintErr("Verifying before Scavenge...");
     heap_->Verify();
     OS::PrintErr(" done.\n");
   }
@@ -402,7 +402,7 @@ void Scavenger::Scavenge(bool invoke_api_callbacks) {
   }
 
   if (FLAG_verify_after_gc) {
-    OS::PrintErr("Verifying after Scavenge... ");
+    OS::PrintErr("Verifying after Scavenge...");
     heap_->Verify();
     OS::PrintErr(" done.\n");
   }