Remove the ability in dart2js to turn off syntax-only generic methods.

tests/compiler/dart2js/options_helper.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library options_helper;
 
 import 'package:compiler/src/options.dart';
 export 'package:compiler/src/options.dart';
 
 class MockDiagnosticOptions implements DiagnosticOptions {
   const MockDiagnosticOptions();
 
   bool get fatalWarnings => false;
   bool get terseDiagnostics => false;
   bool get suppressWarnings => false;
   bool get suppressHints => false;
   bool get showAllPackageWarnings => false;
   bool get hidePackageWarnings => true;
   bool showPackageWarningsFor(Uri uri) => false;
 }
-
-class MockParserOptions implements ParserOptions {
-  bool get enableGenericMethodSyntax => true;
-}