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

runtime/vm/dart_api_impl.cc

 // 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.
 
 #include "include/dart_api.h"
 #include "include/dart_mirrors_api.h"
 #include "include/dart_native_api.h"
 
 #include "platform/assert.h"
 #include "lib/stacktrace.h"
@@ skipping 1358 matching lines @@
 DART_EXPORT bool Dart_ShouldPauseOnStart() {
   Isolate* isolate = Isolate::Current();
   CHECK_ISOLATE(isolate);
   return isolate->message_handler()->should_pause_on_start();
 }
 
 
 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);
 }
 
 
 DART_EXPORT bool Dart_IsPausedOnStart() {
   Isolate* isolate = Isolate::Current();
   CHECK_ISOLATE(isolate);
   return isolate->message_handler()->is_paused_on_start();
 }
 
@@ skipping 4731 matching lines @@
   return Api::Success();
 }
 #endif  // DART_PRECOMPILER
 
 
 DART_EXPORT bool Dart_IsRunningPrecompiledCode() {
   return Dart::IsRunningPrecompiledCode();
 }
 
 }  // namespace dart