Fix lint warnings.

Author: rkistner

example/custom_functions_example.dart

@@ -1,9 +1,7 @@
-import 'dart:ffi';
 import 'dart:io';
 import 'dart:isolate';
 
 import 'package:sqlite_async/sqlite_async.dart';
-import 'package:sqlite3/open.dart' as sqlite_open;
 import 'package:sqlite3/sqlite3.dart' as sqlite;
 
 /// Since the functions need to be created on every SQLite connection,

example/json_example.dart

@@ -63,6 +63,7 @@ RETURNING id''', [users]);
   var ids = idRows.map((row) => row['id']).toList();
 
   // Alternatively, using json1 functions directly.
+  // ignore: unused_local_variable
   var idRows2 = await db.execute('''
 INSERT INTO users(name, email)
 SELECT e.value ->> 'name', e.value ->> 'email' FROM json_each(?) e

lib/src/connection_pool.dart

@@ -26,6 +26,7 @@ class SqliteConnectionPool with SqliteQueries implements SqliteConnection {
 
   final Mutex mutex;
 
+  @override
   bool closed = false;
 
   /// Open a new connection pool.

lib/src/sqlite_connection_impl.dart

@@ -44,6 +44,7 @@ class SqliteConnectionImpl with SqliteQueries implements SqliteConnection {
     await _isolateClient.ready;
   }
 
+  @override
   bool get closed {
     return _isolateClient.closed;
   }

lib/src/sqlite_database.dart

@@ -104,6 +104,7 @@ class SqliteDatabase with SqliteQueries implements SqliteConnection {
     await _initialized;
   }
 
+  @override
   bool get closed {
     return _pool.closed;
   }

scripts/benchmark.dart

@@ -60,13 +60,13 @@ List<SqliteBenchmark> benchmarks = [
   }, maxBatchSize: 10000, enabled: true),
   SqliteBenchmark('Write lock',
       (SqliteDatabase db, List<List<String>> parameters) async {
-    for (var params in parameters) {
+    for (var _ in parameters) {
       await db.writeLock((tx) async {});
     }
   }, maxBatchSize: 5000, enabled: false),
   SqliteBenchmark('Read lock',
       (SqliteDatabase db, List<List<String>> parameters) async {
-    for (var params in parameters) {
+    for (var _ in parameters) {
       await db.readLock((tx) async {});
     }
   }, maxBatchSize: 5000, enabled: false),