Fix some API documentation bugs in new pause on start exit APIs

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
diff --git a/runtime/vm/dart_api_impl.cc b/runtime/vm/dart_api_impl.cc
index 3c4cb1789dfdcc513acd3a3f2c959f3113ffe670..673c54f8270b52b7c65ab7ac0b66d8867b92e59f 100644
--- a/runtime/vm/dart_api_impl.cc
+++ b/runtime/vm/dart_api_impl.cc
@@ -1376,6 +1376,10 @@ DART_EXPORT bool Dart_ShouldPauseOnStart() {
 DART_EXPORT void Dart_SetShouldPauseOnStart(bool should_pause) {
   Isolate* isolate = Isolate::Current();
   CHECK_ISOLATE(isolate);
+  if (isolate->is_runnable()) {
+    FATAL1("%s expects the current isolate to not be runnable yet.",
+           CURRENT_FUNC);
+  }
   return isolate->message_handler()->set_should_pause_on_start(should_pause);
 }
 
@@ -1406,6 +1410,10 @@ DART_EXPORT bool Dart_ShouldPauseOnExit() {
 DART_EXPORT void Dart_SetShouldPauseOnExit(bool should_pause) {
   Isolate* isolate = Isolate::Current();
   CHECK_ISOLATE(isolate);
+  if (isolate->is_runnable()) {
+    FATAL1("%s expects the current isolate to not be runnable yet.",
+           CURRENT_FUNC);

[turnidge, 2016/02/04 17:13:38]

I could see allowing this to be set late.

[Cutch, 2016/02/04 17:26:58]

Done and comment updated.

+  }
   return isolate->message_handler()->set_should_pause_on_exit(should_pause);
 }