Add command line history to Observatory debugger.

runtime/observatory/lib/src/cli/command.dart

Index: runtime/observatory/lib/src/cli/command.dart
diff --git a/runtime/observatory/lib/src/cli/command.dart b/runtime/observatory/lib/src/cli/command.dart
index c1da71bc772f50e34ab9ef911ed0c58f87836e33..30a78748218935bbb376cf627530e07b19fb9f33 100644
--- a/runtime/observatory/lib/src/cli/command.dart
+++ b/runtime/observatory/lib/src/cli/command.dart
@@ -151,6 +151,7 @@ class RootCommand extends _CommandBase {
 
   // Runs a command.
   Future runCommand(String line) {
+    _historyAdvance(line);
     var args = _splitLine(line);
     var commands =  _match(args, true);
     if (commands.isEmpty) {
@@ -174,6 +175,39 @@ class RootCommand extends _CommandBase {
     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';
   }