connectivity_plus_linux: downgrade the dbus dependency

Temporarily pin to an older version of the dbus package (with slightly
different API) until canonical/dbus.dart#111 has been solved.

Author: jpnurmi

packages/connectivity_plus_linux/lib/src/network_manager.dart

@@ -28,9 +28,14 @@ class NetworkManager extends DBusRemoteObject {
   }
 
   Stream<String> subscribeTypeChanged() {
-    return subscribePropertiesChanged()
-        .where((event) => event.changedProperties.containsKey(_kType))
-        .map((event) => (event.changedProperties[_kType] as DBusString).value);
+    final controller = StreamController<String>();
+    subscribePropertiesChanged((_, changedProperties, __) {
+      final type = changedProperties[_kType];
+      if (type != null) {
+        controller.add((type as DBusString).value);
+      }
+    });
+    return controller.stream;
   }
 
   Future<NMConnection> createConnection() {

packages/connectivity_plus_linux/pubspec.yaml

@@ -12,7 +12,7 @@ dependencies:
   flutter:
     sdk: flutter
   connectivity_plus_platform_interface: ^0.2.0
-  dbus: 0.0.0-dev.20
+  dbus: ">=0.0.0-dev.17 <=0.0.0-dev.19"
   meta: ^1.2.3
 
 dev_dependencies: