Use the package_resolver package.

test/io.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.
 
 // TODO(nweiz): Remove this tag when we can get [packageDir] working without it
 // (dart-lang/sdk#24022).
 library test.test.io;
 
 import 'dart:async';
 import 'dart:io';
+import 'dart:isolate';
 
+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';
 
 /// The path to the root directory of the `test` package.
-final String packageDir = p.dirname(p.dirname(libraryPath(#test.test.io)));
+final Future<String> packageDir = PackageResolver.current.packagePath('test');
 
 /// The path to the `pub` executable in the current Dart SDK.
 final _pubPath = p.absolute(p.join(
     p.dirname(Platform.resolvedExecutable),
     Platform.isWindows ? 'pub.bat' : 'pub'));
 
 /// The platform-specific message emitted when a nonexistent file is loaded.
 final String noSuchFileMessage = Platform.isWindows
     ? "The system cannot find the file specified."
     : "No such file or directory";
@@ skipping 70 matching lines @@
 /// Runs the test executable with the package root set properly.
 ///
 /// If [forwardStdio] is true, the standard output and error from the process
 /// will be printed as part of the parent test. This is used for debugging.
 ScheduledProcess runTest(List<String> args, {String reporter,
     int concurrency, Map<String, String> environment,
     bool forwardStdio: false}) {
   concurrency ??= 1;
 
   var allArgs = [
-    p.absolute(p.join(packageDir, 'bin/test.dart')),
-    "--package-root=${p.join(packageDir, 'packages')}",
+    packageDir.then((dir) => p.absolute(p.join(dir, 'bin/test.dart'))),
     "--concurrency=$concurrency"
   ];
   if (reporter != null) allArgs.add("--reporter=$reporter");
   allArgs.addAll(args);
 
   if (environment == null) environment = {};
   environment.putIfAbsent("_DART_TEST_TESTING", () => "true");
 
   var process = runDart(allArgs,
       environment: environment,
       description: "dart bin/test.dart");
 
   if (forwardStdio) {
     process.stdoutStream().listen(print);
     process.stderrStream().listen(print);
   }
 
   return process;
 }
 
 /// Runs Dart.
 ScheduledProcess runDart(List<String> args, {Map<String, String> environment,
     String description}) {
-  var allArgs = Platform.executableArguments.map((arg) {
-    // The package root might be relative, so we need to make it absolute if
-    // we're going to run in a different working directory.
-    if (!arg.startsWith("--package-root=")) return arg;
-    return "--package-root=" +
-        p.absolute(p.fromUri(arg.substring("--package-root=".length)));
-  }).toList()..addAll(args);
+  var allArgs = Platform.executableArguments
+      .where((arg) =>
+          !arg.startsWith("--package-root=") && !arg.startsWith("--packages="))
+      .toList()
+      ..add(PackageResolver.current.processArgument)
+      ..addAll(args);
 
   return new ScheduledProcess.start(
       p.absolute(Platform.resolvedExecutable), allArgs,
       workingDirectory: _sandbox,
       environment: environment,
       description: description);
 }
 
 /// Runs Pub.
 ScheduledProcess runPub(List args, {Map<String, String> environment}) {
@@ skipping 22 matching lines @@
     var match;
     while (match == null) {
       var line = await pub.stdout.next();
       match = _servingRegExp.firstMatch(line);
     }
     _pubServePortCompleter.complete(int.parse(match[1]));
   }, "waiting for pub serve to emit its port number");
 
   return pub;
 }