Produce useful error messages for empty pubspecs.

sdk/lib/_internal/pub/test/get/path/empty_pubspec_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS d.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 d.file.
 
-import 'package:path/path.dart' as path;
+import 'package:path/path.dart' as p;
 
+import '../../../lib/src/exit_codes.dart' as exit_codes;
 import '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 
 main() {
   initConfig();
-  integration('path dependency when path is a file', () {
+  // Regression test for issue 20103.
+  integration('path dependency to an empty pubspec', () {
     d.dir('foo', [
       d.libDir('foo'),
-      d.libPubspec('foo', '0.0.1')
+      d.file('pubspec.yaml', '')
     ]).create();
 
-    d.file('dummy.txt', '').create();
-    var dummyPath = path.join(sandboxDir, 'dummy.txt');
-
     d.dir(appPath, [
       d.appPubspec({
-        "foo": {"path": dummyPath}
+        "foo": {"path": "../foo"}
       })
     ]).create();
 
-    pubGet(error: 'Path dependency for package foo must refer to a '
-                  'directory, not a file. Was "$dummyPath".');
+    pubGet(exitCode: exit_codes.DATA, error:
+        'Error on line 1, column 1 of ${p.join('..', 'foo', 'pubspec.yaml')}: '
+            'Missing the required "name" field.');
   });
 }