Fixed issue in MallocHooks where a MallocHookScope was accidentally removed during a merge, causing…

runtime/vm/object_test.cc

Index: runtime/vm/object_test.cc
diff --git a/runtime/vm/object_test.cc b/runtime/vm/object_test.cc
index c435602c24a15d7c8f053d718f1c4ef163dd04eb..fb4b621d61096674bfa36cd4d717a7a71813f2a5 100644
--- a/runtime/vm/object_test.cc
+++ b/runtime/vm/object_test.cc
@@ -10,6 +10,7 @@
 #include "vm/dart_entry.h"
 #include "vm/debugger.h"
 #include "vm/isolate.h"
+#include "vm/malloc_hooks.h"
 #include "vm/object.h"
 #include "vm/object_store.h"
 #include "vm/simulator.h"
@@ -2709,6 +2710,9 @@ ISOLATE_UNIT_TEST_CASE(Code) {
 // Test for immutability of generated instructions. The test crashes with a
 // segmentation fault when writing into it.
 ISOLATE_UNIT_TEST_CASE(CodeImmutability) {
+  bool stack_trace_collection_enabled =
+      MallocHooks::stack_trace_collection_enabled();
+  MallocHooks::set_stack_trace_collection_enabled(false);
   extern void GenerateIncrement(Assembler * assembler);
   Assembler _assembler_;
   GenerateIncrement(&_assembler_);
@@ -2726,6 +2730,8 @@ ISOLATE_UNIT_TEST_CASE(CodeImmutability) {
     // TODO(regis, fschneider): Should this be FATAL() instead?
     OS::DebugBreak();
   }
+  MallocHooks::set_stack_trace_collection_enabled(
+      stack_trace_collection_enabled);
 }