Don't consider --profile-vm when collecting allocation samples; introduce a separate --profile-vm-al

runtime/vm/profiler.cc

Index: runtime/vm/profiler.cc
diff --git a/runtime/vm/profiler.cc b/runtime/vm/profiler.cc
index 6eaca60e7084d8d77263a15ccd48d9581818e10e..72319adf8bf01bc98550fdcb26139a96401ca432 100644
--- a/runtime/vm/profiler.cc
+++ b/runtime/vm/profiler.cc
@@ -1085,7 +1085,8 @@ void Profiler::SampleAllocation(Thread* thread, intptr_t cid) {
   Sample* sample = SetupSample(thread, sample_buffer, os_thread->trace_id());
   sample->SetAllocationCid(cid);
 
-  if (FLAG_profile_vm) {
+  // TODO(flutter/flutter#10744): Enable native stack walker.
+  if (FLAG_profile_vm && false) {

[zra, 2017/06/15 20:50:21]

Should we add a separate flag for this instead that is turned off in the Flutter
engine?

Maybe --profile_allocation_vm or --profile_vm_allocation. Not sure what a good
name would be.

[rmacnak, 2017/07/11 18:17:18]

So even with good frame pointer, using the native stack walker doesn't produce
the traces we intended when introduce allocation tracing because the Dart code
is hidden behind too many C++ frames to show up in the profile. Removed the
reference to the frame pointers in Flutter Android bug and introduced a separate
flag as Zach suggests.

     ProfilerNativeStackWalker native_stack_walker(
         (isolate != NULL) ? isolate->main_port() : ILLEGAL_PORT, sample,
         sample_buffer, stack_lower, stack_upper, pc, fp, sp);