Fixes how to lookup sdk dir to support tests deeper inside test/ folders.

pkg/code_transformers/lib/tests.dart

 // Copyright (c) 2014, 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.
 
 /// Collection of utilities which are useful for creating unit tests for
 /// Barback transformers.
 library code_transformers.tests;
 
 import 'dart:async' show Future;
+import 'dart:io' show Platform;
 
 import 'package:barback/barback.dart' show Transformer;
+import 'package:path/path.dart' as path;
 import 'package:unittest/unittest.dart';
 
 import 'src/test_harness.dart';
+import 'src/dart_sdk.dart';
 
 /// Defines a test which invokes [applyTransformers].
 testPhases(String testName, List<List<Transformer>> phases,
     Map<String, String> inputs, Map<String, String> results,
     [List<String> messages]) {
   test(testName,
     () => applyTransformers(phases, inputs: inputs, results: results,
         messages: messages));
 }
 
 /// Updates the provided transformers with [inputs] as asset inputs then
 /// validates that [results] were generated.
 ///
 /// The keys for inputs and results are 'package_name|lib/file.dart'.
 /// Only files which are specified in results are validated.
 ///
 /// If [messages] is non-null then this will validate that only the specified
 /// messages were generated, ignoring info messages.
 Future applyTransformers(List<List<Transformer>> phases,
     {Map<String, String> inputs: const {},
     Map<String, String> results: const {},
     List<String> messages: const []}) {
 
   var helper = new TestHelper(phases, inputs, messages)..run();
   return helper.checkAll(results).then((_) => helper.tearDown());
 }
+
+/// Variant of [dartSdkDirectory] which includes additional cases only
+/// typically encountered in Dart's testing environment.
+String get testingDartSdkDirectory {
+  var sdkDir = dartSdkDirectory;
+  if (sdkDir == null) {
+    // If we cannot find the SDK dir, then assume this is being run from Dart's
+    // source directory and this script is the main script.
+    var segments = path.split(path.fromUri(Platform.script));
+    var index = segments.indexOf('pkg');
+    expect(index, greaterThan(0),
+        reason: 'testingDartSdkDirectory is only supported in pkg/ tests');
+    sdkDir = path.joinAll(segments.sublist(0, index)..add('sdk'));
+  }
+  return sdkDir;
+}