Build errors against recent valac

I'd like to build Geary with support for Unity indicators in the Geary Flatpak package, but it's failing to build against recent valac:

mjog@blanchefort:~/Projects/GNOME/libunity_7.1.4+16.04.20180209.1.orig$ vala --version
Vala 0.44.0
mjog@blanchefort:~/Projects/GNOME/libunity_7.1.4+16.04.20180209.1.orig$ make
make all-recursive
make[1]: Entering directory '/home/mjog/Projects/GNOME/libunity_7.1.4+16.04.20180209.1.orig'
Making all in data
make[2]: Entering directory '/home/mjog/Projects/GNOME/libunity_7.1.4+16.04.20180209.1.orig/data'
  ITMRG com.canonical.Unity.Lenses.gschema.xml
  GEN com.canonical.Unity.Lenses.gschema.valid
make[2]: Leaving directory '/home/mjog/Projects/GNOME/libunity_7.1.4+16.04.20180209.1.orig/data'
Making all in protocol
make[2]: Entering directory '/home/mjog/Projects/GNOME/libunity_7.1.4+16.04.20180209.1.orig/protocol'
  GEN libunity_protocol_private_la_vala.stamp
Command-line option `--thread` is deprecated and will be ignored
protocol-scope-discovery.vala:24.3-24.40: warning: the modifier `static' is not applicable to constants
  private static const string SCOPES_DIR = "unity/scopes";
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
protocol-scope-discovery.vala:190.5-190.43: warning: the modifier `static' is not applicable to constants
    private static const string SCOPE_GROUP = "Scope";
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
protocol-scope-discovery.vala:191.5-191.45: warning: the modifier `static' is not applicable to constants
    private static const string DESKTOP_GROUP = "Desktop Entry";
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
protocol-scope-discovery.vala:902.5-902.49: warning: the modifier `static' is not applicable to constants
    private static const string SCOPE_GROUP_GROUP = "Scope Group";
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
protocol-preview-player.vala:41.3-41.46: warning: the modifier `static' is not applicable to constants
  static const string PREVIEW_PLAYER_DBUS_NAME = "com.canonical.Unity.Lens.Music.PreviewPlayer";
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
protocol-preview-player.vala:42.3-42.46: warning: the modifier `static' is not applicable to constants
  static const string PREVIEW_PLAYER_DBUS_PATH = "/com/canonical/Unity/Lens/Music/PreviewPlayer";
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
protocol-scope-interface.vala:117.3-117.51: warning: DBus methods are recommended to throw at least `GLib.Error' or `GLib.DBusError, GLib.IOError'
  public abstract async ActivationReplyRaw activate (
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
protocol-scope-interface.vala:124.3-124.57: warning: DBus methods are recommended to throw at least `GLib.Error' or `GLib.DBusError, GLib.IOError'
  public abstract async HashTable<string, Variant> search (
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
protocol-scope-interface.vala:134.7-134.46: error: Synchronous out-parameters are not supported in async methods
      out HashTable<string, Variant> out_hints,
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
protocol-scope-interface.vala:130.3-130.43: warning: DBus methods are recommended to throw at least `GLib.Error' or `GLib.DBusError, GLib.IOError'
  public abstract async string open_channel (
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
protocol-scope-interface.vala:137.3-137.42: warning: DBus methods are recommended to throw at least `GLib.Error' or `GLib.DBusError, GLib.IOError'
  public abstract async void close_channel (
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
protocol-scope-interface.vala:142.3-142.63: warning: DBus methods are recommended to throw at least `GLib.Error' or `GLib.DBusError, GLib.IOError'
  public abstract async HashTable<string, Variant> push_results (
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
protocol-scope-interface.vala:151.3-151.42: warning: DBus methods are recommended to throw at least `GLib.Error' or `GLib.DBusError, GLib.IOError'
  public abstract async void set_view_type (uint view_type) throws IOError;
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
unity-scope-proxy-remote.vala:379.12-379.21: error: The name `channel_id' does not exist in the context of `Unity.Protocol.ScopeProxyRemote.open_channel'
    return channel_id;
           ^^^^^^^^^^
Compilation failed: 2 error(s), 12 warning(s)
make[2]: *** [Makefile:958: libunity_protocol_private_la_vala.stamp] Error 1
make[2]: Leaving directory '/home/mjog/Projects/GNOME/libunity_7.1.4+16.04.20180209.1.orig/protocol'
make[1]: *** [Makefile:572: all-recursive] Error 1
make[1]: Leaving directory '/home/mjog/Projects/GNOME/libunity_7.1.4+16.04.20180209.1.orig'
make: *** [Makefile:476: all] Error 2