Add command line history to Observatory debugger.

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.
 List<String> _splitLine(String line) {
   var args = line.split(' ').where((arg) {
       return arg != ' ' && arg != '';
@@ skipping 133 matching lines @@
         .then((localCompletions) {
           completions.addAll(localCompletions);
           return completions;
         });
     }
     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');
     } 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');
     }
   }
 
   // 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('');
     }
     return _match(args, preferExact);
   }
 
+  // Command line history always contains one slot to hold the current
+  // line, so we start off with one entry.
+  List<String> history = [''];
+  int historyPos = 0;
+
+  String historyPrev(String line) {
+    if (historyPos == 0) {
+      return line;
+    }
+    history[historyPos] = line;
+    historyPos--;
+    return history[historyPos];
+  }
+
+  String historyNext(String line) {
+    if (historyPos == history.length - 1) {
+      return line;
+    }
+    history[historyPos] = line;
+    historyPos++;
+    return history[historyPos];
+  }
+
+  void _historyAdvance(String line) {
+    // Replace the last history line.
+    historyPos = history.length - 1;
+    history[historyPos] = line;
+
+    // Create an empty spot for the next line.
+    history.add('');
+    historyPos++;
+  }
+
   Future run(List<String> args) {
     throw 'should-not-execute-the-root-command';
   }
 
   toString() => 'RootCommand';
 }
 
 // A node in the command tree.
 abstract class Command extends _CommandBase {
   Command(this.name, List<Command> children) : super(children);
 
   final String name;
   String get fullName {
     if (_parent is RootCommand) {
       return name;
     } else {
       Command parent = _parent;
       return '${parent.fullName} $name';
     }
   }
 
   toString() => 'Command(${name})';
 }