Move _fatal from dart:core to dart:_internal

runtime/lib/async_patch.dart

Index: runtime/lib/async_patch.dart
diff --git a/runtime/lib/async_patch.dart b/runtime/lib/async_patch.dart
index 3e22fb044545fef55933c31361b6ea48c946e976..6cc9a16abac123436bd676170c76cf9a690f3929 100644
--- a/runtime/lib/async_patch.dart
+++ b/runtime/lib/async_patch.dart
@@ -97,8 +97,8 @@ class _AsyncStarStreamController {
   // suspend;
   // if (controller.isCancelled) return;
   bool add(event) {
-    if (!onListenReceived) _fatal("yield before stream is listened to!");
-    if (isSuspendedAtYield) _fatal("unexpected yield");
+    if (!onListenReceived) fatal("yield before stream is listened to!");
+    if (isSuspendedAtYield) fatal("unexpected yield");
     // If stream is cancelled, tell caller to exit the async generator.
     if (!controller.hasListener) {
       return true;
@@ -115,7 +115,7 @@ class _AsyncStarStreamController {
   // Returns true if the caller should terminate
   // execution of the generator.
   bool addStream(Stream stream) {
-    if (!onListenReceived) _fatal("yield before stream is listened to!");
+    if (!onListenReceived) fatal("yield before stream is listened to!");
     // If stream is cancelled, tell caller to exit the async generator.
     if (!controller.hasListener) return true;
     isAdding = true;