Ensure packageRoot is non-null in docgen.

dart/pkg/docgen/lib/src/generator.dart

Index: dart/pkg/docgen/lib/src/generator.dart
diff --git a/dart/pkg/docgen/lib/src/generator.dart b/dart/pkg/docgen/lib/src/generator.dart
index 2f1722c87cce4f8109084c2e2a38160878f60f74..7df02600fd71def4975451c2698f20f7711a0342 100644
--- a/dart/pkg/docgen/lib/src/generator.dart
+++ b/dart/pkg/docgen/lib/src/generator.dart
@@ -274,9 +274,10 @@ Future<MirrorSystem> analyzeLibraries(List<Uri> libraries, String
       ..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) {