Use the package_resolver package.

lib/src/runner/browser/compiler_pool.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.
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:async/async.dart';
+import 'package:package_resolver/package_resolver.dart';
 import 'package:path/path.dart' as p;
 import 'package:pool/pool.dart';
 
 import '../../util/io.dart';
 import '../configuration.dart';
 import '../load_exception.dart';
 
 /// A regular expression matching the first status line printed by dart2js.
 final _dart2jsStatus =
     new RegExp(r"^Dart file \(.*\) compiled to JavaScript: .*\n?");
@@ skipping 17 matching lines @@
   /// The memoizer for running [close] exactly once.
   final _closeMemo = new AsyncMemoizer();
 
   /// Creates a compiler pool that multiple instances of `dart2js` at once.
   CompilerPool(Configuration config)
       : _pool = new Pool(config.concurrency),
         _config = config;
 
   /// Compile the Dart code at [dartPath] to [jsPath].
   ///
-  /// This wraps the Dart code in the standard browser-testing wrapper. If
-  /// [packageRoot] is provided, it's used as the package root for the
-  /// compilation.
+  /// This wraps the Dart code in the standard browser-testing wrapper.
   ///
   /// The returned [Future] will complete once the `dart2js` process completes
   /// *and* all its output has been printed to the command line.
-  Future compile(String dartPath, String jsPath, {String packageRoot}) {
+  Future compile(String dartPath, String jsPath) {
     return _pool.withResource(() {
       if (_closed) return null;
 
       return withTempDir((dir) async {
         var wrapperPath = p.join(dir, "runInBrowser.dart");
         new File(wrapperPath).writeAsStringSync('''
           import "package:stream_channel/stream_channel.dart";
 
           import "package:test/src/runner/plugin/remote_platform_helpers.dart";
           import "package:test/src/runner/browser/post_message_channel.dart";
 
           import "${p.toUri(p.absolute(dartPath))}" as test;
 
           main(_) async {
             var channel = serializeSuite(() => test.main, hidePrints: false);
             postMessageChannel().pipe(channel);
           }
         ''');
 
         var dart2jsPath = _config.dart2jsPath;
         if (Platform.isWindows) dart2jsPath += '.bat';
 
-        var args = ["--checked", wrapperPath, "--out=$jsPath"]
-          ..addAll(_config.dart2jsArgs);
-
-        if (packageRoot != null) {
-          args.add("--package-root=${p.toUri(p.absolute(packageRoot))}");
-        }
+        var args = [
+          "--checked",
+          wrapperPath,
+          "--out=$jsPath",
+          await PackageResolver.current.processArgument
+        ]..addAll(_config.dart2jsArgs);
 
         if (_config.color) args.add("--enable-diagnostic-colors");
 
         var process = await Process.start(dart2jsPath, args);
         if (_closed) {
           process.kill();
           return;
         }
 
         _processes.add(process);
@@ skipping 55 matching lines @@
   /// have been killed and all resources released.
   Future close() {
     return _closeMemo.runOnce(() async {
       await Future.wait(_processes.map((process) async {
         process.kill();
         await process.exitCode;
       }));
     });
   }
 }