Add dart2js option to select library categories.

dart/tests/compiler/dart2js/dart_backend_test.dart

 // Copyright (c) 2012, 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.
 
 import 'dart:async';
 import 'dart:uri';
 import 'parser_helper.dart';
 import 'mock_compiler.dart';
 import '../../../sdk/lib/_internal/compiler/compiler.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/dart2jslib.dart' as leg;
@@ skipping 100 matching lines @@
     return new Future.immediate('');
   }
 
   handler(uri, begin, end, message, kind) {
     if (identical(kind, Diagnostic.ERROR) || identical(kind, Diagnostic.CRASH)) {
       Expect.fail('$uri: $begin-$end: $message [$kind]');
     }
   }
 
   final options = <String>['--output-type=dart'];
+  // Some tests below are using dart:io.
+  options.add('--categories=Client,Server');
   if (minify) options.add('--minify');
   if (stripTypes) options.add('--force-strip=types');
 
   compile(
       scriptUri,
       fileUri('libraryRoot/'),
       fileUri('packageRoot/'),
       provider,
       handler,
       options).then(continuation);
@@ skipping 621 matching lines @@
   testStaticAccessIoLib();
   testLocalFunctionPlaceholder();
   testMinification();
   testClosureLocalsMinified();
   testParametersMinified();
   testDeclarationTypePlaceholders();
   testPlatformLibraryMemberNamesAreFixed();
   testConflictsWithCoreLib();
   testUnresolvedNamedConstructor();
 }