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

runtime/vm/profiler.h

Index: runtime/vm/profiler.h
diff --git a/runtime/vm/profiler.h b/runtime/vm/profiler.h
index 8ffa4ecd097c0eaff0276e37607f3a4f66b60d7f..05ecedd2b67421943901f911f1e88df63fb8580c 100644
--- a/runtime/vm/profiler.h
+++ b/runtime/vm/profiler.h
@@ -54,6 +54,15 @@ class Profiler : public AllStatic {
   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);