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 94476ee86f126465fdbf54122d6061d6ebaef14d..ab5391b03aa4a024d0d1d043d010a28ee64e9503 100644
--- a/runtime/vm/scavenger.cc
+++ b/runtime/vm/scavenger.cc
@@ -382,7 +382,7 @@ 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");
   }
@@ -404,7 +404,7 @@ 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");
   }