Drop an unnecessary generic on a method (#1440)

The method is only called from one place with a consistently typed
argument, so the generics are not buying value. Remove them to make the
code simpler.
diff --git a/pkgs/test_core/CHANGELOG.md b/pkgs/test_core/CHANGELOG.md
index 41d52e5..ce4d85f 100644
--- a/pkgs/test_core/CHANGELOG.md
+++ b/pkgs/test_core/CHANGELOG.md
@@ -1,3 +1,5 @@
+## 0.3.15-dev
+
 ## 0.3.14
 
 * Handle issue closing `stdin` during shutdown.
diff --git a/pkgs/test_core/lib/src/runner.dart b/pkgs/test_core/lib/src/runner.dart
index ede16c1..69b1ac5 100644
--- a/pkgs/test_core/lib/src/runner.dart
+++ b/pkgs/test_core/lib/src/runner.dart
@@ -8,7 +8,6 @@
 import 'dart:io';
 
 import 'package:async/async.dart';
-
 import 'package:test_api/src/backend/group.dart'; // ignore: implementation_imports
 import 'package:test_api/src/backend/group_entry.dart'; // ignore: implementation_imports
 import 'package:test_api/src/backend/operating_system.dart'; // ignore: implementation_imports
@@ -20,7 +19,6 @@
 import 'package:test_api/src/utils.dart'; // ignore: implementation_imports
 import 'package:test_core/src/runner/reporter/multiplex.dart';
 
-import 'util/io.dart';
 import 'runner/application_exception.dart';
 import 'runner/configuration.dart';
 import 'runner/configuration/reporters.dart';
@@ -32,6 +30,8 @@
 import 'runner/reporter.dart';
 import 'runner/reporter/compact.dart';
 import 'runner/reporter/expanded.dart';
+import 'runner/runner_suite.dart';
+import 'util/io.dart';
 
 /// A class that loads and runs tests based on a [Configuration].
 ///
@@ -368,7 +368,7 @@
   /// index. This makes the tests pretty tests across shards, and since the
   /// tests are continuous, makes us more likely to be able to re-use
   /// `setUpAll()` logic.
-  T _shardSuite<T extends Suite>(T suite) {
+  RunnerSuite _shardSuite(RunnerSuite suite) {
     if (_config.totalShards == null) return suite;
 
     var shardSize = suite.group.testCount / _config.totalShards;
@@ -382,7 +382,7 @@
       return count >= shardStart && count < shardEnd;
     });
 
-    return filtered as T;
+    return filtered;
   }
 
   /// Loads each suite in [suites] in order, pausing after load for runtimes
diff --git a/pkgs/test_core/pubspec.yaml b/pkgs/test_core/pubspec.yaml
index 87d4be2..260c625 100644
--- a/pkgs/test_core/pubspec.yaml
+++ b/pkgs/test_core/pubspec.yaml
@@ -1,5 +1,5 @@
 name: test_core
-version: 0.3.14
+version: 0.3.15-dev
 description: A basic library for writing tests and running them on the VM.
 homepage: https://github.com/dart-lang/test/blob/master/pkgs/test_core