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

pkgs/oauth2/lib/src/client.dart

@@ -73,7 +73,7 @@ class Client extends http.BaseClient {
   final bool _basicAuth;
 
   /// The underlying HTTP client.
-  http.Client? _httpClient;
+  http.Client _httpClient;
 
   /// Creates a new client from a pre-existing set of credentials.
   ///
@@ -111,7 +111,7 @@ class Client extends http.BaseClient {
     }
 
     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;
@@ -181,7 +181,6 @@ class Client extends http.BaseClient {
   /// Closes this client and its underlying HTTP client.
   @override
   void close() {
-    _httpClient?.close();
-    _httpClient = null;
+    _httpClient.close();
   }
 }

pkgs/oauth2/test/client_test.dart

@@ -189,6 +189,24 @@ 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,
+      );
+
+      client.close();
+
+      expect(
+        client.read(requestUri),
+        throwsA(const TypeMatcher<http.ClientException>()),
+      );
+    });
   });
 
   group('with invalid credentials', () {