Revert "Revert "Use native async/await support in pub.""

sdk/lib/_internal/pub_generated/lib/src/source/unknown.dart

Index: sdk/lib/_internal/pub_generated/lib/src/source/unknown.dart
diff --git a/sdk/lib/_internal/pub_generated/lib/src/source/unknown.dart b/sdk/lib/_internal/pub_generated/lib/src/source/unknown.dart
deleted file mode 100644
index a3049cb5e05783191f8419ae5f7cf10262ce70f1..0000000000000000000000000000000000000000
--- a/sdk/lib/_internal/pub_generated/lib/src/source/unknown.dart
+++ /dev/null
@@ -1,48 +0,0 @@
-// Copyright (c) 2014, 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.source.unknown;
-
-import 'dart:async';
-
-import '../package.dart';
-import '../pubspec.dart';
-import '../source.dart';
-
-/// A [Null Object] that represents a source not recognized by pub.
-///
-/// It provides some default behavior so that pub can work with sources it
-/// doesn't recognize.
-///
-/// [null object]: http://en.wikipedia.org/wiki/Null_Object_pattern
-class UnknownSource extends Source {
-  final String name;
-
-  UnknownSource(this.name);
-
-  /// Two unknown sources are the same if their names are the same.
-  bool operator ==(other) => other is UnknownSource && other.name == name;
-
-  int get hashCode => name.hashCode;
-
-  Future<Pubspec> doDescribe(PackageId id) =>
-      throw new UnsupportedError(
-          "Cannot describe a package from unknown source '$name'.");
-
-  Future get(PackageId id, String symlink) =>
-      throw new UnsupportedError("Cannot get an unknown source '$name'.");
-
-  /// Returns the directory where this package can be found locally.
-  Future<String> getDirectory(PackageId id) =>
-      throw new UnsupportedError(
-          "Cannot find a package from an unknown source '$name'.");
-
-  bool descriptionsEqual(description1, description2) =>
-      description1 == description2;
-
-  /// Unknown sources do no validation.
-  dynamic parseDescription(String containingPath, description,
-      {bool fromLockFile: false}) =>
-      description;
-}