ContextBuilder should to throw when a root folder does not exist.

pkg/analyzer/lib/src/context/builder.dart

Index: pkg/analyzer/lib/src/context/builder.dart
diff --git a/pkg/analyzer/lib/src/context/builder.dart b/pkg/analyzer/lib/src/context/builder.dart
index 58301b7680db3f7264409ea157ab33823c2646ad..ba6926cd38f4e1b25f6f5cb7d455cef53de7d8b2 100644
--- a/pkg/analyzer/lib/src/context/builder.dart
+++ b/pkg/analyzer/lib/src/context/builder.dart
@@ -522,8 +522,9 @@ class ContextBuilder {
   Resource _findPackagesLocation(String path) {
     Folder folder = resourceProvider.getFolder(path);
     if (!folder.exists) {
-      throw new ArgumentError.value(path, "path", "Directory does not exist.");
+      return null;
     }
+
     File checkForConfigFile(Folder folder) {
       File file = folder.getChildAssumingFile('.packages');
       if (file.exists) {