Async-step, first cut.

runtime/observatory/lib/src/cli/command.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.
 
 part of cli;
 
 // Splits a line into a list of string args.  Each arg retains any
 // trailing whitespace so that we can reconstruct the original command
 // line from the pieces.
 List<String> _splitLine(String line) {
@@ skipping 152 matching lines @@
     return new Future.value(completions);
   }
 
   // Runs a command.
   Future runCommand(String line) {
     _historyAdvance(line);
     var args = _splitLine(line);
     var commands =  _match(args, true);
     if (commands.isEmpty) {
       // TODO(turnidge): Add a proper exception class for this.
-      return new Future.error('notfound');
+      return new Future.error('No such command');
     } else if (commands.length == 1) {
       return commands[0].run(args.sublist(commands[0]._depth));
     } else {
       // TODO(turnidge): Add a proper exception class for this.
-      return new Future.error('ambiguous');
+      return new Future.error('Ambiguous command');
     }
   }
 
   // Find all matching commands.  Useful for implementing help systems.
   List<Command> matchCommand(List<String> args, bool preferExact) {
     if (args.isEmpty) {
       // Adding an empty string to the end causes us to match all
       // subcommands of the last command.
       args.add('');
     }
@@ skipping 51 matching lines @@
     if (_parent is RootCommand) {
       return name;
     } else {
       Command parent = _parent;
       return '${parent.fullName} $name';
     }
   }
 
   toString() => 'Command(${name})';
 }