Fix a bug in pubspec parsing.

test/pubspec_test.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 pubspec_test;
 
 import 'dart:async';
 
 import 'package:pub/src/package.dart';
 import 'package:pub/src/pubspec.dart';
@@ skipping 211 matching lines @@
 
     test("throws if version is not a version constraint", () {
       expectPubspecException('''
 dependencies:
   foo:
     mock: ok
     version: not constraint
 ''', (pubspec) => pubspec.dependencies);
     });
 
+    test("throws if there's no source", () {
+      expectPubspecException('''
+dependencies:
+  foo:
+    version: 1.2.3
+''', (pubspec) => pubspec.dependencies);
+    });
+
     test("throws if 'name' is not a string", () {
       expectPubspecException('name: [not, a, string]',
           (pubspec) => pubspec.name);
     });
 
     test("throws if version is not a string", () {
       expectPubspecException('version: [2, 0, 0]',
           (pubspec) => pubspec.version,
           '"version" field must be a string');
     });
@@ skipping 280 matching lines @@
       test("uses the key if the value is null", () {
         var pubspec = new Pubspec.parse('''
 executables:
   command:
 ''', sources);
         expect(pubspec.executables['command'], equals('command'));
       });
     });
   });
 }