[oauth2] Add isClosed and exception handling for closed state to oaut…

pkgs/oauth2/lib/src/client.dart

@@ -66,6 +66,9 @@ class Client extends http.BaseClient {
   Credentials get credentials => _credentials;
   Credentials _credentials;
 
+  /// Indicates whether the client is closed or not.
+  bool get isClosed => _httpClient == null;
+
   /// Callback to be invoked whenever the credentials refreshed.
   final CredentialsRefreshedCallback? _onCredentialsRefreshed;
 
@@ -110,8 +113,13 @@ class Client extends http.BaseClient {
       await refreshCredentials();
     }
 
+    final httpClient = _httpClient;
+    if (httpClient == null) {
+      throw http.ClientException('Client is already closed.');
+    }
+
     request.headers['authorization'] = 'Bearer ${credentials.accessToken}';
-    var response = await _httpClient!.send(request);
+    var response = await httpClient.send(request);
 
     if (response.statusCode != 401) return response;
     if (!response.headers.containsKey('www-authenticate')) return response;

pkgs/oauth2/test/client_test.dart

@@ -189,6 +189,26 @@ void main() {
 
       expect(client.refreshCredentials(), throwsA(isStateError));
     });
+
+    test("won't send a request with closed client", () {
+      var credentials = oauth2.Credentials('access token');
+
+      var client = oauth2.Client(
+        credentials,
+        identifier: 'identifier',
+        secret: 'secret',
+        httpClient: httpClient,
+      );
+
+      expect(client.isClosed, equals(false));
+      client.close();
+      expect(client.isClosed, equals(true));
+
+      expect(
+        client.read(requestUri),
+        throwsA(const TypeMatcher<http.ClientException>()),
+      );
+    });
   });
 
   group('with invalid credentials', () {