Use the package_resolver package.

test/runner/precompiled_paths_test.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.
 
 @TestOn("vm")
 @Tags(const ["chrome"])
 
 import 'dart:io';
 
+import 'package:package_resolver/package_resolver.dart';
 import 'package:path/path.dart' as p;
 import 'package:scheduled_test/descriptor.dart' as d;
 import 'package:scheduled_test/scheduled_process.dart';
 import 'package:scheduled_test/scheduled_stream.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
 import 'package:test/src/util/io.dart';
 
 import '../io.dart';
 
@@ skipping 12 matching lines @@
         var channel = serializeSuite(() {
           return () => test("success", () {});
         }, hidePrints: false);
         postMessageChannel().pipe(channel);
       }
     """).create();
 
     d.dir("precompiled", []).create();
 
     var dart2js = new ScheduledProcess.start(p.join(sdkDir, 'bin', 'dart2js'), [
-      "--package-root=${p.join(p.current, 'packages')}",
+      PackageResolver.current.processArgument,
       "to_precompile.dart",
       "--out=precompiled/test.dart.browser_test.dart.js"
     ], workingDirectory: sandbox);
     dart2js.shouldExit(0);
 
     d.file("test.dart", "invalid dart}").create();
 
     var test = runTest(
         ["-p", "chrome", "--precompiled=precompiled/", "test.dart"]);
     test.stdout.expect(containsInOrder([
       "+0: success",
       "+1: All tests passed!"
     ]));
     test.shouldExit(0);
   });
 }