Fix unused imports and unnecessary casts (#735)

lib/src/frontend/never_called.dart

@@ -62,6 +62,6 @@ T Function<T>(
                     ? " no arguments."
                     : ":\n${bullet(arguments.map(prettyPrint))}")),
         zone.run(() => new Chain.current()));
-    return null as T;
+    return null;
   };
 }

lib/src/runner/plugin/platform_helpers.dart

@@ -15,7 +15,6 @@ import '../../backend/test_platform.dart';
 import '../../util/io.dart';
 import '../../util/remote_exception.dart';
 import '../../util/stack_trace_mapper.dart';
-import '../application_exception.dart';
 import '../configuration.dart';
 import '../configuration/suite.dart';
 import '../environment.dart';

test/frontend/never_called_test.dart

@@ -5,7 +5,6 @@
 import 'dart:async';
 
 import 'package:term_glyph/term_glyph.dart' as glyph;
-import 'package:test/src/backend/state.dart';
 import 'package:test/test.dart';
 
 import '../utils.dart';