[vm] Don't enable the profiler by default. Enable the profiler at startup with --observe, or later …

runtime/vm/profiler.h

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef RUNTIME_VM_PROFILER_H_
 #define RUNTIME_VM_PROFILER_H_
 
 #include "vm/allocation.h"
 #include "vm/bitfield.h"
 #include "vm/code_observers.h"
@@ skipping 36 matching lines @@
   // Count of failed checks:
   int64_t failure_native_allocation_sample;
 };
 
 class Profiler : public AllStatic {
  public:
   static void InitOnce();
   static void InitAllocationSampleBuffer();
   static void Shutdown();
 
+#if !defined(PRODUCT)
+  static void EnsureEnabled() {

[zra, 2017/07/31 15:03:11]

It looks like this is only used for tests, so think the definition should
probably go in the source file.

[rmacnak, 2017/07/31 21:12:13]

Done.

+    if (!FLAG_profiler) {
+      FLAG_profiler = true;
+      Profiler::InitOnce();
+    }
+  }
+#endif
+
   static void SetSampleDepth(intptr_t depth);
   static void SetSamplePeriod(intptr_t period);
 
   static SampleBuffer* sample_buffer() { return sample_buffer_; }
   static AllocationSampleBuffer* allocation_sample_buffer() {
     return allocation_sample_buffer_;
   }
 
   static void DumpStackTrace(void* context);
   static void DumpStackTrace(bool for_crash = true);
@@ skipping 628 matching lines @@
  private:
   ZoneGrowableArray<ProcessedSample*> samples_;
   CodeLookupTable* code_lookup_table_;
 
   DISALLOW_COPY_AND_ASSIGN(ProcessedSampleBuffer);
 };
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_PROFILER_H_