Async-step, first cut.

runtime/observatory/lib/src/elements/debugger.dart

 // Copyright (c) 2014, 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.
 
 library debugger_page_element;
 
 import 'dart:async';
 import 'dart:html';
 import 'observatory_element.dart';
 import 'package:observatory/cli.dart';
@@ skipping 331 matching lines @@
   String helpShort =
       'Continue running the isolate until it reaches the next source location';
 
   String helpLong =
       'Continue running the isolate until it reaches the next source '
       'location.\n'
       '\n'
       'Syntax: step\n';
 }
 
+class AsyncStepCommand extends DebuggerCommand {
+  AsyncStepCommand(Debugger debugger) : super(debugger, 'astep', []) {
+  }
+
+  Future run(List<String> args) async {
+    if (debugger.isolatePaused()) {
+      var event = debugger.isolate.pauseEvent;
+      if (event.asyncContinuation == null) {
+        debugger.console.print("No async continuation at this location");
+        return;
+      }
+      var bpt = await
+          debugger.isolate.addBreakOnActivation(event.asyncContinuation);
+      return debugger.isolate.resume();
+    } else {
+      debugger.console.print('The program is already running');
+    }
+  }
+
+  String helpShort =
+      'Step into asynchronous continuation';
+
+  String helpLong =
+      'Continue running the isolate until control returns to the current '
+      'activation of an async or async* function.\n'
+      '\n'
+      'Syntax: astep\n';
+}
+
 class FinishCommand extends DebuggerCommand {
   FinishCommand(Debugger debugger) : super(debugger, 'finish', []);
 
   Future run(List<String> args) {
     if (debugger.isolatePaused()) {
       return debugger.isolate.stepOut();
     } else {
       debugger.console.print('The program is already running');
       return new Future.value(null);
     }
@@ skipping 581 matching lines @@
     cmd = new RootCommand([
         new HelpCommand(this),
         new PrintCommand(this),
         new DownCommand(this),
         new UpCommand(this),
         new FrameCommand(this),
         new PauseCommand(this),
         new ContinueCommand(this),
         new NextCommand(this),
         new StepCommand(this),
+        new AsyncStepCommand(this),
         new FinishCommand(this),
         new BreakCommand(this),
         new SetCommand(this),
         new ClearCommand(this),
         new DeleteCommand(this),
         new InfoCommand(this),
         new IsolateCommand(this),
         new RefreshCommand(this),
     ]);
     _stdioPrinter = new _VMStreamPrinter(this);
@@ skipping 132 matching lines @@
         } else if (event.exception != null) {
           console.print('Paused due to exception at '
                         '${script.name}:${line}:${col}');
           // This seems to be missing if we are paused-at-exception after
           // paused-at-isolate-exit. Maybe we shutdown part of the debugger too
           // soon?
           console.printRef(event.exception);
         } else {
           console.print('Paused at ${script.name}:${line}:${col}');
         }
+        if (event.asyncContinuation != null) {
+          console.print("Paused in async function: 'astep' available");
+        }
       });
     }
   }
 
   Future _reportBreakpointEvent(ServiceEvent event) {
     var bpt = event.breakpoint;
     var verb = null;
     switch (event.kind) {
       case ServiceEvent.kBreakpointAdded:
         verb = 'added';
@@ skipping 736 matching lines @@
       });
   }
 
   void focus() {
     $['textBox'].focus();
   }
 
   DebuggerInputElement.created() : super.created();
 }