Improve updating following and fix potential crash.

test/channel_test.dart

@@ -0,0 +1,115 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:venera/utils/channel.dart';
+
+void main() {
+  test("1-1-1", () async {
+    var channel = Channel<int>(1);
+    await channel.push(1);
+    var item = await channel.pop();
+    expect(item, 1);
+  });
+
+  test("1-3-1", () async {
+    var channel = Channel<int>(1);
+
+    // producer
+    () async {
+      await channel.push(1);
+    }();
+    () async {
+      await channel.push(2);
+    }();
+    () async {
+      await channel.push(3);
+    }();
+
+    // consumer
+    var results = <int>[];
+    for (var i = 0; i < 3; i++) {
+      var item = await channel.pop();
+      if (item != null) {
+        results.add(item);
+      }
+    }
+    expect(results.length, 3);
+  });
+
+  test("2-3-1", () async {
+    var channel = Channel<int>(2);
+
+    // producer
+    () async {
+      await channel.push(1);
+    }();
+    () async {
+      await channel.push(2);
+    }();
+    () async {
+      await channel.push(3);
+    }();
+
+    // consumer
+    var results = <int>[];
+    for (var i = 0; i < 3; i++) {
+      var item = await channel.pop();
+      if (item != null) {
+        results.add(item);
+      }
+    }
+    expect(results.length, 3);
+  });
+
+  test("1-1-3", () async {
+    var channel = Channel<int>(1);
+
+    // producer
+    () async {
+      print("push 1");
+      await channel.push(1);
+      print("push 2");
+      await channel.push(2);
+      print("push 3");
+      await channel.push(3);
+      print("push done");
+      channel.close();
+    }();
+
+    // consumer
+    var consumers = <Future>[];
+    var results = <int>[];
+    for (var i = 0; i < 3; i++) {
+      consumers.add(() async {
+        while (true) {
+          var item = await channel.pop();
+          if (item == null) {
+            break;
+          }
+          print("pop $item");
+          results.add(item);
+        }
+      }());
+    }
+
+    await Future.wait(consumers);
+    expect(results.length, 3);
+  });
+
+  test("close", () async {
+    var channel = Channel<int>(2);
+
+    // producer
+    () async {
+      await channel.push(1);
+      await channel.push(2);
+      await channel.push(3);
+      channel.close();
+    }();
+
+    // consumer
+    await channel.pop();
+    await channel.pop();
+    await channel.pop();
+    var item4 = await channel.pop();
+    expect(item4, null);
+  });
+}