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

utils/pub/entrypoint.dart

Index: utils/pub/entrypoint.dart
diff --git a/utils/pub/entrypoint.dart b/utils/pub/entrypoint.dart
index 2758707385ebd577c65b46c00a9819d6009fb562..02a2359414f08d39bbf13c462fa0860ec890688b 100644
--- a/utils/pub/entrypoint.dart
+++ b/utils/pub/entrypoint.dart
@@ -233,16 +233,17 @@ class Entrypoint {
   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}").';
     });
   }
 }