Enable and fix unnecessary_ignore lint (#2456)

Author: kevmoo

analysis_options.yaml

@@ -19,3 +19,4 @@ analyzer:
 linter:
   rules:
     - avoid_private_typedef_functions
+    - unnecessary_ignore

pkgs/test/lib/src/runner/browser/compilers/compiler_support.dart

@@ -11,7 +11,7 @@ import 'package:shelf/shelf.dart' as shelf;
 import 'package:test_api/backend.dart' show StackTraceMapper, SuitePlatform;
 import 'package:test_core/src/runner/configuration.dart'; // ignore: implementation_imports
 import 'package:test_core/src/runner/suite.dart'; // ignore: implementation_imports
-import 'package:web_socket_channel/web_socket_channel.dart'; // ignore: implementation_imports
+import 'package:web_socket_channel/web_socket_channel.dart';
 
 /// The shared interface for all compiler support libraries.
 abstract class CompilerSupport {

pkgs/test_core/lib/src/runner/reporter/json.dart

@@ -9,9 +9,7 @@ import 'dart:io' show pid;
 import 'package:collection/collection.dart';
 import 'package:path/path.dart' as p;
 import 'package:stack_trace/stack_trace.dart';
-import 'package:test_api/hooks.dart' // ignore: implementation_imports
-    show
-        TestFailure;
+import 'package:test_api/hooks.dart' show TestFailure;
 import 'package:test_api/src/backend/compiler.dart'; // ignore: implementation_imports
 import 'package:test_api/src/backend/group.dart'; // ignore: implementation_imports
 import 'package:test_api/src/backend/live_test.dart'; // ignore: implementation_imports

pkgs/test_core/lib/src/runner/vm/environment.dart

@@ -7,7 +7,7 @@ import 'dart:async';
 import 'package:async/async.dart';
 import 'package:vm_service/vm_service.dart';
 
-import '../environment.dart'; // ignore: implementation_imports
+import '../environment.dart';
 
 /// The environment in which VM tests are loaded.
 class VMEnvironment implements Environment {