Fix a bug in pubspec parsing.

lib/src/pubspec.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 pub.pubspec;
 
 import 'package:path/path.dart' as path;
 import 'package:pub_semver/pub_semver.dart';
 import 'package:source_span/source_span.dart';
 import 'package:yaml/yaml.dart';
@@ skipping 445 matching lines @@
 
         if (spec.containsKey('version')) {
           spec.remove('version');
           versionConstraint = _parseVersionConstraint(
               specNode.nodes['version']);
         }
 
         var sourceNames = spec.keys.toList();
         if (sourceNames.length > 1) {
           _error('A dependency may only have one source.', specNode.span);
+        } else if (sourceNames.isEmpty) {
+          _error('A dependency must contain a source.', specNode.span);
         }
 
         sourceName = sourceNames.single;
         if (sourceName is! String) {
           _error('A source name must be a string.',
               specNode.nodes.keys.single.span);
         }
 
         descriptionNode = specNode.nodes[sourceName];
       } else {
@@ skipping 101 matching lines @@
     implements ApplicationException {
   PubspecException(String message, SourceSpan span)
       : super(message, span);
 }
 
 /// Returns whether [uri] is a file URI.
 ///
 /// This is slightly more complicated than just checking if the scheme is
 /// 'file', since relative URIs also refer to the filesystem on the VM.
 bool _isFileUri(Uri uri) => uri.scheme == 'file' || uri.scheme == '';