Gracefully handle pubspecs with dependencies using unknown sources.

sdk/lib/_internal/pub/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 'package:unittest/unittest.dart';
 
 import '../lib/src/pubspec.dart';
 import '../lib/src/source.dart';
@@ skipping 63 matching lines @@
     });
 
     test("allows an empty dev dependencies map", () {
       var pubspec = new Pubspec.parse(null, '''
 dev_dependencies:
 ''', sources);
 
       expect(pubspec.devDependencies, isEmpty);
     });
 
+    test("allows an unknown source", () {
+      var pubspec = new Pubspec.parse(null, '''
+dependencies:
+  foo:
+    unknown: blah
+''', sources);
+
+      var foo = pubspec.dependencies[0];
+      expect(foo.name, equals('foo'));
+      expect(foo.source, equals('unknown'));
+    });
+
     test("throws if a package is in dependencies and dev_dependencies", () {
       expectFormatError('''
 dependencies:
   foo:
     mock: ok
 dev_dependencies:
   foo:
     mock: ok
 ''');
     });
@@ skipping 109 matching lines @@
 
       test("throws if the sdk isn't a valid version constraint", () {
         expectFormatError('''
 environment:
   sdk: "oopies"
 ''');
       });
     });
   });
 }