Add necessary support functions so that embedders can implemented pause on start and exit

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index dc99e4c65d4aee132e51ac929e291fd4f1ad6041..b83e75251d1acabc040efc0fd9ee3966e9fab9f5 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -779,6 +779,7 @@ Isolate::Isolate(const Dart_IsolateFlags& api_flags)
       message_handler_(NULL),
       spawn_state_(NULL),
       is_runnable_(false),
+      pause_isolates_flags_overridden_(false),
       gc_prologue_callback_(NULL),
       gc_epilogue_callback_(NULL),
       defer_finalization_count_(0),
@@ -1069,8 +1070,10 @@ bool Isolate::MakeRunnable() {
   ASSERT(object_store()->root_library() != Library::null());
   set_is_runnable(true);
   if (!ServiceIsolate::IsServiceIsolate(this)) {
-    message_handler()->set_pause_on_start(FLAG_pause_isolates_on_start);
-    message_handler()->set_pause_on_exit(FLAG_pause_isolates_on_exit);
+    if (!pause_isolates_flags_overridden_) {
+      message_handler()->set_pause_on_start(FLAG_pause_isolates_on_start);
+      message_handler()->set_pause_on_exit(FLAG_pause_isolates_on_exit);
+    }

[turnidge, 2016/02/03 21:33:36]

As discussed offline, we may be able to remove pause_isolates_flags_overridden_.

We would move the initialization of pause_on_start/pause_on_exit to the
constructor for the message handler.  Then, inside the message handler, we could
use the runnable bit to determine whether or not we should look at the value...

[turnidge, 2016/02/03 21:35:53]

One more thing, we wouldn't want pause_on_start to ever happen for native
message handlers, so we would probably guard the whole thing in an isolate() !=
NULL...

[Cutch, 2016/02/03 23:04:04]

Acknowledged.

[Cutch, 2016/02/03 23:04:05]

Done.

     if (FLAG_pause_isolates_on_unhandled_exceptions) {
       debugger()->SetExceptionPauseInfo(kPauseOnUnhandledExceptions);
     }