Add an intrinsified early out path for Dart timeline calls

runtime/vm/intrinsifier_arm.cc

Index: runtime/vm/intrinsifier_arm.cc
diff --git a/runtime/vm/intrinsifier_arm.cc b/runtime/vm/intrinsifier_arm.cc
index e878d033bdc1b7c73d269f0bc34330f742dae26c..bde5e3f7f1078060f3f51deeaab978a0bb265d44 100644
--- a/runtime/vm/intrinsifier_arm.cc
+++ b/runtime/vm/intrinsifier_arm.cc
@@ -15,6 +15,7 @@
 #include "vm/object_store.h"
 #include "vm/regexp_assembler.h"
 #include "vm/symbols.h"
+#include "vm/timeline.h"
 
 namespace dart {
 
@@ -2104,6 +2105,24 @@ void Intrinsifier::Profiler_getCurrentTag(Assembler* assembler) {
   __ Ret();
 }
 
+
+void Intrinsifier::Timeline_isDartStreamEnabled(Assembler* assembler) {
+  Label true_label;
+  // Load TimelineStream*.
+  __ ldr(R0, Address(THR, Thread::dart_stream_offset()));
+  // Load uintptr_t from TimelineStream*.
+  __ ldr(R0, Address(R0, TimelineStream::enabled_offset()));

[rmacnak, 2016/05/16 23:10:52]

This would deference NULL if it ran in product mode.

if (!FLAG_support_timeline) {
  __ LoadObject(R0, Bool::False());
  __ Ret();
  return;

  or 
  __break();

}

// check thread->dart_stream()->enabled()

[Cutch, 2016/05/17 15:47:47]

I had ensured all calls to isDartStreamEnabled come after checks for product
mode but I've incorporated your suggestion here because it's safer.

+  __ cmp(R0, Operand(0));
+  __ b(&true_label, NE);
+  // Not enabled.
+  __ LoadObject(R0, Bool::False());
+  __ Ret();
+  // Enabled.
+  __ Bind(&true_label);
+  __ LoadObject(R0, Bool::True());
+  __ Ret();
+}
+
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_ARM