msglist: Handle updated events in MessageListView (zulip#118).

Processes an UpdateMessageEvent and hands it off to the MessageListView to update, if the message is visible in the MessageListView. This completes the changes required for issue zulip#118.

lib/model/message_list.dart

@@ -1,5 +1,6 @@
 import 'package:flutter/foundation.dart';
 
+import '../api/model/events.dart';
 import '../api/model/model.dart';
 import '../api/route/messages.dart';
 import 'content.dart';
@@ -86,6 +87,53 @@ class MessageListView extends ChangeNotifier {
     notifyListeners();
   }
 
+  Message applyChangesToMessage(UpdateMessageEvent event, Message oldMessage) {
+    Map<String, dynamic> oldMessageJson = oldMessage.toJson();
+
+    //TODO: What other fields should be edited by the update event?
+    oldMessageJson["content"] = event.renderedContent;
+    oldMessageJson["lastEditTimestamp"] = event.editTimestamp;
+
+    return Message.fromJson(oldMessageJson);
+  }
+
+  ///This is almost directly copied from package:collection/algorithms.dart.
+  ///The way that package was set up doesn't allow us to search
+  ///for a message ID among a bunch of message objects - this is a quick
+  ///modification of that method to work here for us.
+  int findMessageWithId(int messageId) {
+  var min = 0;
+  var max = messages.length;
+
+  while (min < max) {
+    var mid = min + ((max - min) >> 1);
+    Message message = messages[mid];
+    var comp = message.id.compareTo(messageId);
+    if (comp == 0) return mid;
+    if (comp < 0) {
+      min = mid + 1;
+    } else {
+      max = mid;
+    }
+  }
+  return -1;
+}
+
+  void maybeUpdateMessage(UpdateMessageEvent event) {
+    int oldIdx = findMessageWithId(event.messageId);
+
+    if (oldIdx == -1)  {
+      return;
+    }
+
+    Message oldMessage = messages[oldIdx];
+    Message newMessage = applyChangesToMessage(event, oldMessage);
+
+    messages.replaceRange(oldIdx, oldIdx+1, [newMessage]);
+    contents.replaceRange(oldIdx, oldIdx+1, [parseContent(newMessage.content)]);
+    notifyListeners();
+  }
+
   /// Called when the app is reassembled during debugging, e.g. for hot reload.
   ///
   /// This will redo from scratch any computations we can, such as parsing

lib/model/store.dart

@@ -272,7 +272,9 @@ class PerAccountStore extends ChangeNotifier {
       }
     } else if (event is UpdateMessageEvent) {
       assert(debugLog("server event: update_message ${event.messageId}"));
-      // TODO handle
+      for (final view in _messageListViews) {
+        view.maybeUpdateMessage(event);
+      }
     } else if (event is DeleteMessageEvent) {
       assert(debugLog("server event: delete_message ${event.messageIds}"));
       // TODO handle