Gracefully handle pubspecs with dependencies using unknown sources.

sdk/lib/_internal/pub/test/pubspec_test.dart

Index: sdk/lib/_internal/pub/test/pubspec_test.dart
diff --git a/sdk/lib/_internal/pub/test/pubspec_test.dart b/sdk/lib/_internal/pub/test/pubspec_test.dart
index 19f62d4e1bfc4b5f69894b5494e1b7c02ac352e3..45413dbbea97303d98fa1d57a66d8df73e3abcfb 100644
--- a/sdk/lib/_internal/pub/test/pubspec_test.dart
+++ b/sdk/lib/_internal/pub/test/pubspec_test.dart
@@ -81,6 +81,18 @@ dev_dependencies:
       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: