Experimenting with using a file chooser in a Chrome App written in Dart. Got stuck again :(

file_chooser_app.dart

import 'dart:async';
import 'dart:chrome';
import 'dart:html';
import 'package:js/js.dart' as js;

void main() {
  var open = new ButtonElement()..text = 'Open';
  document.body.children.add(open);
  
  write(s) => document.body.children.add(new DivElement()..text = s);
  
  open.onClick.listen((_) {
    FileEntry.choose()
      .then((entry) {
        entry.displayPath.then(write);
        // Yikes
        // Can't use dart's file reader as entry is a proxy not a Dart FileEntry/Blob.
        // Not sure how to call the javascript ctor new FileReader() via js_interop.
        //var reader = new FileReader();
        //reader.readAsText(entry);
      });
  });
}

class ChooseEntryType {
  const ChooseEntryType(this._value);
  final String _value;
  toString() => _value;
}

const OPEN_FILE = const ChooseEntryType("openFile");
const OPEN_FILE_WRITABLE = const ChooseEntryType("openWritableFile");
const SAVE_FILE = const ChooseEntryType("saveFile");

class AcceptOption {
  AcceptOption(this.description, this.mimeTypes, this.extensions);
  final String description;
  final List<String> mimeTypes;
  final List<String> extensions;
}

class FileEntry {
  
  final _proxy;
  
  FileEntry._internal(this._proxy);
  
  void release() {
    js.release(_proxy);
  }
  
  static Future<FileEntry> choose(
      {ChooseEntryType type: OPEN_FILE,
       String suggestedName : '',
       List<AcceptOption> accepts : const [],
       bool acceptsAllTypes: true}) {
    
    var cpl = new Completer<String>();
    js.scoped(() {
      var options = {'type': type.toString(),
                     'suggestedName': suggestedName,
                     'accepts': js.array(accepts), //TODO
                     'acceptsAllTypes': acceptsAllTypes};
      js.context.chrome.fileSystem.chooseEntry(js.map(options),
          new js.Callback.once((fileEntry) {
            cpl.complete(new FileEntry._internal(fileEntry));
          }));
    });
    return cpl.future;
  }
  
  static FileEntry getById(String id) {
    var fileEntryProxy;
    js.scoped(() {      
      fileEntryProxy = js.context.chrome.fileSystem.getEntryById(id);
      js.retain(fileEntryProxy);
    });
    return new FileEntry._internal(fileEntryProxy);
  }
  
  String get id {
    var entryId;
    js.scoped(() {
      entryId = js.context.chrome.fileSystem.getEntryId(_proxy);
    });
    return entryId;
  }
  
  Future<String> get displayPath {
    var cpl = new Completer<String>();
    
    js.scoped(() {      
      js.context.chrome.fileSystem.getDisplayPath(_proxy,
          new js.Callback.once((path) {
            cpl.complete(path);
          }));
    });
    
    return cpl.future;
  }
  
  Future<FileEntry> makeWritable() {
    throw new UnimplementedError();
  }
  
  Future<bool> get isWritable {
    throw new UnimplementedError();
  }
}