Improve the error message for pub install without a name key in the pubspec.

utils/pub/entrypoint.dart

 // Copyright (c) 2012, 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('entrypoint');
 
 #import('io.dart');
 #import('lock_file.dart');
 #import('package.dart');
 #import('root_source.dart');
@@ skipping 215 matching lines @@
     });
   }
 
   /**
    * Validate that the pubspec for the entrypoint exists and specifies the name
    * of the root package.
    */
   Future _validatePubspec() {
     var future = new Future.immediate(null);;
     if (root.pubspec.isEmpty) {
-      future = exists(join(path, "pubspec.yaml")).transform((exists) {
+      future = exists(join(root.dir, "pubspec.yaml")).transform((exists) {
         if (exists) return;
-        throw 'Could not find a file named "pubspec.yaml" in the directory'
+        throw 'Could not find a file named "pubspec.yaml" in the directory '
           '$path.';
       });
     }
 
     return future.transform((_) {
       if (root.pubspec.name != null) return;
-      throw '"pubspec.yaml" must contain a "name" key.';
+      throw '"pubspec.yaml" is missing the required "name" field (e.g. "name: '
+        '${root.name}").';
     });
   }
 }