Correctly cancel subscription in database (#198)

Fixes #121

Author: Eddydpyl

packages/firebase-dart/lib/src/database.dart

@@ -309,7 +309,7 @@ class Query<T extends database_interop.QueryJsImpl> extends JsObjectWrapper<T> {
     }
 
     void stopListen() {
-      jsObject.off(eventType);
+      jsObject.off(eventType, callbackWrap);
     }
 
     streamController = new StreamController<QueryEvent>.broadcast(

packages/firebase-dart/lib/src/interop/database_interop.dart

@@ -62,7 +62,10 @@ abstract class QueryJsImpl {
   external bool isEqual(QueryJsImpl other);
   external QueryJsImpl limitToFirst(int limit);
   external QueryJsImpl limitToLast(int limit);
-  external void off([String eventType, Func2Opt1 callback, context]);
+  external void off(
+      [String eventType,
+      Func2Opt1<DataSnapshotJsImpl, String, Null> callback,
+      context]);
   external Func0 on(
       String eventType, Func2Opt1<DataSnapshotJsImpl, String, Null> callback,
       [cancelCallbackOrContext, context]);

packages/firebase-dart/test/database_test.dart

@@ -135,6 +135,34 @@ void main() {
         expect(val, "Cooking delicious dinner!");
       });
 
+      test("distinctListeners", () async {
+        var sub1 = ref.onValue.listen(expectAsync1((event) {
+          // print("first listener called.");
+        }, count: 1));
+
+        addTearDown(sub1.cancel);
+
+        var sub2 = ref.onValue.listen(expectAsync1((event) {
+          // print("second listener called.");
+        }, count: 0));
+
+        await sub2.cancel().then((_) => print("second listener cancelled."));
+
+        var anotherRef = database.ref("test");
+        var sub3 = anotherRef.onValue.listen(expectAsync1((event) {
+          // print("third listener called.");
+        }, count: 1));
+
+        addTearDown(sub3.cancel);
+
+        var yetAnotherRef = database.ref("test");
+        var sub4 = yetAnotherRef.onValue.listen(expectAsync1((event) {
+          // print("fourth listener called.");
+        }, count: 0));
+
+        await sub4.cancel().then((_) => print("fourth listener cancelled."));
+      });
+
       test("onValue", () async {
         var childRef = ref.child("todos");
         // ignore: unawaited_futures