Add an intrinsified early out path for Dart timeline calls

runtime/vm/intrinsifier_x64.cc

Index: runtime/vm/intrinsifier_x64.cc
diff --git a/runtime/vm/intrinsifier_x64.cc b/runtime/vm/intrinsifier_x64.cc
index 402613e37fab085efe2d3f7afb0c038e62d0ac58..1c0acee1500fa4f21a671ebf088505843164ddb9 100644
--- a/runtime/vm/intrinsifier_x64.cc
+++ b/runtime/vm/intrinsifier_x64.cc
@@ -14,6 +14,7 @@
 #include "vm/object_store.h"
 #include "vm/regexp_assembler.h"
 #include "vm/symbols.h"
+#include "vm/timeline.h"
 
 namespace dart {
 
@@ -2086,6 +2087,29 @@ void Intrinsifier::Profiler_getCurrentTag(Assembler* assembler) {
   __ ret();
 }
 
+
+void Intrinsifier::Timeline_isDartStreamEnabled(Assembler* assembler) {
+  if (!FLAG_support_timeline) {
+    __ LoadObject(RAX, Bool::False());
+    __ ret();
+    return;
+  }
+  Label true_label;
+  // Load TimelineStream*.
+  __ movq(RAX, Address(THR, Thread::dart_stream_offset()));
+  // Load uintptr_t from TimelineStream*.
+  __ movq(RAX, Address(RAX, TimelineStream::enabled_offset()));
+  __ cmpq(RAX, Immediate(0));
+  __ j(NOT_ZERO, &true_label, Assembler::kNearJump);
+  // Not enabled.
+  __ LoadObject(RAX, Bool::False());
+  __ ret();
+  // Enabled.
+  __ Bind(&true_label);
+  __ LoadObject(RAX, Bool::True());
+  __ ret();
+}
+
 #undef __
 
 }  // namespace dart