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 d37c2d76ac2effa682675213c78c5428ba41f75e..527a84c8e197b9042b2bed75c187416ac7c925cb 100644
--- a/runtime/vm/dart.cc
+++ b/runtime/vm/dart.cc
@@ -23,6 +23,7 @@
 
 namespace dart {
 
+DECLARE_FLAG(bool, heap_trace);
 DECLARE_FLAG(bool, print_bootstrap);
 DECLARE_FLAG(bool, print_class_table);
 DECLARE_FLAG(bool, trace_isolates);
@@ -98,6 +99,9 @@ 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;
 }
 
@@ -174,6 +178,9 @@ RawError* Dart::InitializeIsolate(const uint8_t* snapshot_buffer, void* data) {
   }
 
   StubCode::Init(isolate);
+  if (FLAG_heap_trace) {
+    isolate->heap()->trace()->Init(isolate);
+  }
   isolate->heap()->EnableGrowthControl();
   isolate->set_init_callback_data(data);
   if (FLAG_print_class_table) {