licenses: Add [additionalLicenses], to be used for a font soon

For a proposed UI to show the user the list of licenses tracked by
[LicenseRegistry], see PR zulip#101.

Author: chrisbobbe

lib/licenses.dart

@@ -0,0 +1,13 @@
+import 'package:flutter/foundation.dart';
+
+/// Our [LicenseEntryCollector] for licenses that aren't included by default.
+///
+/// Licenses that ship with our Dart-package dependencies are included
+/// automatically. This collects other licenses, such as for fonts we include in
+/// our asset bundle.
+// If the license text is meant to be read from a file in the asset bundle,
+// remember to include the file in the asset bundle by listing its path
+// under `assets` in pubspec.yaml.
+Stream<LicenseEntry> additionalLicenses() async* {
+  // TODO add some
+}

lib/main.dart

@@ -1,5 +1,7 @@
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 
+import 'licenses.dart';
 import 'log.dart';
 import 'widgets/app.dart';
 
@@ -8,5 +10,6 @@ void main() {
     debugLogEnabled = true;
     return true;
   }());
+  LicenseRegistry.addLicense(additionalLicenses);
   runApp(const ZulipApp());
 }