Debugger support for step-into async and async* functions.

runtime/lib/async_patch.dart

 // Copyright (c) 2015, 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.
 
 import "dart:_internal" hide Symbol;
 
 // Equivalent of calling FATAL from C++ code.
 _fatal(msg) native "DartAsync_fatal";
 
 // We need to pass the value as first argument and leave the second and third
@@ skipping 55 matching lines @@
 // Called as part of the 'await for (...)' construct. Registers the
 // awaiter on the stream.
 void _asyncStarListenHelper(var object, var awaiter) {
   if (object is! _StreamImpl) {
     return;
   }
   // `object` is a `_StreamImpl`.
   object._awaiter = awaiter;
 }
 
+void _asyncStarMoveNextHelper(var stream) {
+  if (stream is! _StreamImpl) {
+    return;
+  }
+  // stream is a _StreamImpl.
+  if (stream._generator == null) {
+    // No generator registered, this isn't an async* Stream.
+    return;
+  }
+  _moveNextDebuggerStepCheck(stream._generator);
+}
+
 // _AsyncStarStreamController is used by the compiler to implement
 // async* generator functions.
 class _AsyncStarStreamController {
   StreamController controller;
   Function asyncStarBody;
   bool isAdding = false;
   bool onListenReceived = false;
   bool isScheduled = false;
   bool isSuspendedAtYield = false;
   Completer cancellationCompleter = null;
@@ skipping 139 matching lines @@
   /// The closure implementing the async[*]-body that is `await`ing this future.
   Function _awaiter;
 
   /// The closure implementing the async-generator body that is creating events
   /// for this stream.
   Function _generator;
 }
 
 /// Returns a [StackTrace] object containing the synchronous prefix for this
 /// asynchronous method.
-Object _asyncStackTraceHelper() native "StackTrace_asyncStackTraceHelper";
+Object _asyncStackTraceHelper(Function async_op)
+    native "StackTrace_asyncStackTraceHelper";
 
 void _clearAsyncThreadStackTrace()
     native "StackTrace_clearAsyncThreadStackTrace";
 
 void _setAsyncThreadStackTrace(StackTrace stackTrace)
     native "StackTrace_setAsyncThreadStackTrace";
+
+void _moveNextDebuggerStepCheck(Function async_op)
+    native "AsyncStarMoveNext_debuggerStepCheck";