Add support for evaluating platform selectors.

bin/unittest.dart

 // Copyright (c) 2015, 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 unittest.unittest;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:isolate';
 
 import 'package:args/args.dart';
 import 'package:stack_trace/stack_trace.dart';
 
+import 'package:unittest/src/backend/test_platform.dart';
 import 'package:unittest/src/runner/reporter/compact.dart';
-import 'package:unittest/src/runner/test_platform.dart';
 import 'package:unittest/src/runner/load_exception.dart';
 import 'package:unittest/src/runner/loader.dart';
 import 'package:unittest/src/util/exit_codes.dart' as exit_codes;
 import 'package:unittest/src/util/io.dart';
 import 'package:unittest/src/utils.dart';
 
 /// The argument parser used to parse the executable arguments.
 final _parser = new ArgParser(allowTrailingOptions: true);
 
 void main(List<String> args) {
@@ skipping 96 matching lines @@
     output = stderr;
   }
 
   output.write("""$message
 
 Usage: pub run unittest:unittest [files or directories...]
 
 ${_parser.usage}
 """);
 }