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

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

 // Copyright (c) 2016, 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 analyzer.src.context.context_builder;
 
 import 'dart:collection';
 import 'dart:core';
 
 import 'package:analyzer/context/declared_variables.dart';
@@ skipping 504 matching lines @@
    * found, starts checking parent directories for `.packages` until reaching
    * the root directory.
    *
    * Return a [File] object representing a `.packages` file if one is found, a
    * [Folder] object for the `packages/` directory if that is found, or `null`
    * if neither is found.
    */
   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) {
         return file;
       }
       return null;
     }
 
     // Check for $cwd/.packages
     File packagesCfgFile = checkForConfigFile(folder);
@@ skipping 167 matching lines @@
   String _readEmbedderYaml(Folder libDir) {
     File file = libDir.getChild(EMBEDDER_FILE_NAME);
     try {
       return file.readAsStringSync();
     } on FileSystemException {
       // File can't be read.
       return null;
     }
   }
 }