Check for null, when packages dir has no parent.

editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/source/PackageUriResolver.java

Index: editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/source/PackageUriResolver.java
diff --git a/editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/source/PackageUriResolver.java b/editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/source/PackageUriResolver.java
index 903b2316d1f1e07aca54d330f9b243add6d322c5..0525dede0d15096af9f19e3a12ad7f44d090764c 100644
--- a/editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/source/PackageUriResolver.java
+++ b/editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/source/PackageUriResolver.java
@@ -173,6 +173,9 @@ public class PackageUriResolver extends UriResolver {
    */
   private boolean isSelfReference(File packagesDir, File file) {
     File rootDir = packagesDir.getParentFile();
+    if (rootDir == null) {
+      return false;
+    }
     String rootPath = rootDir.getAbsolutePath();
     String filePath = file.getAbsolutePath();
     return filePath.startsWith(rootPath + "/lib");