Merge the Merlin heap tracing to top-of-trunk.

runtime/vm/dart.cc

Index: runtime/vm/dart.cc
diff --git a/runtime/vm/dart.cc b/runtime/vm/dart.cc
index a26c0c4521a36d8c7f8b38ee51d4ac483bbd4cb9..4d64bd1ac7560bd1ace1bf902baaac8efd575163 100644
--- a/runtime/vm/dart.cc
+++ b/runtime/vm/dart.cc
@@ -25,6 +25,7 @@ namespace dart {
 
 DEFINE_FLAG(bool, heap_profile_initialize, false,
             "Writes a heap profile on isolate initialization.");
+DECLARE_FLAG(bool, heap_trace);
 DECLARE_FLAG(bool, print_bootstrap);
 DECLARE_FLAG(bool, print_class_table);
 DECLARE_FLAG(bool, trace_isolates);
@@ -99,6 +100,9 @@ const char* Dart::InitOnce(Dart_IsolateCreateCallback create,
   Isolate::SetInterruptCallback(interrupt);
   Isolate::SetUnhandledExceptionCallback(unhandled);
   Isolate::SetShutdownCallback(shutdown);
+  if (FLAG_heap_trace) {
+    HeapTrace::InitOnce(file_open, file_write, file_close);
+  }
   return NULL;
 }
 
@@ -181,6 +185,9 @@ RawError* Dart::InitializeIsolate(const uint8_t* snapshot_buffer, void* data) {
 
   StubCode::Init(isolate);
   isolate->megamorphic_cache_table()->InitMissHandler();
+  if (FLAG_heap_trace) {
+    isolate->heap()->trace()->Init(isolate);
+  }
   isolate->heap()->EnableGrowthControl();
   isolate->set_init_callback_data(data);
   if (FLAG_print_class_table) {