Ensure packageRoot is non-null in docgen.

dart/pkg/docgen/lib/src/generator.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.
 
 library docgen.generator;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:convert';
 import 'dart:io';
@@ skipping 256 matching lines @@
 /// for static inspection of the source code.
 Future<MirrorSystem> analyzeLibraries(List<Uri> libraries, String
     libraryRoot, {String packageRoot}) {
   SourceFileProvider provider = new CompilerSourceFileProvider();
   api.DiagnosticHandler diagnosticHandler = (new FormattingDiagnosticHandler(
       provider)
       ..showHints = false
       ..showWarnings = false).diagnosticHandler;
   Uri libraryUri = new Uri.file(appendSlash(libraryRoot));
   Uri packageUri = null;
-  if (packageRoot != null) {
-    packageUri = new Uri.file(appendSlash(packageRoot));
+  if (packageRoot == null) {
+    packageRoot = Platform.packageRoot;
   }
+  packageUri = new Uri.file(appendSlash(packageRoot));
   return dart2js.analyze(libraries, libraryUri, packageUri,
       provider.readStringFromUri, diagnosticHandler, ['--preserve-comments',
       '--categories=Client,Server'])..catchError((error) {
         logger.severe('Error: Failed to create mirror system. ');
         // TODO(janicejl): Use the stack trace package when bug is resolved.
         // Currently, a string is thrown when it fails to create a mirror
         // system, and it is not possible to use the stack trace. BUG(#11622)
         // To avoid printing the stack trace.
         exit(1);
       });
@@ skipping 187 matching lines @@
   * [Samples](http://www.dartlang.org/samples/)
   * [A Tour of the Dart Libraries](http://www.dartlang.org/docs/dart-up-and-running/contents/ch03.html)
 
 This API reference is automatically generated from the source code in the
 [Dart project](https://code.google.com/p/dart/).
 If you'd like to contribute to this documentation, see
 [Contributing](https://code.google.com/p/dart/wiki/Contributing)
 and
 [Writing API Documentation](https://code.google.com/p/dart/wiki/WritingApiDocumentation).
 """;