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

sdk/lib/_internal/pub_generated/lib/src/barback/cycle_exception.dart

Index: sdk/lib/_internal/pub_generated/lib/src/barback/cycle_exception.dart
diff --git a/sdk/lib/_internal/pub_generated/lib/src/barback/cycle_exception.dart b/sdk/lib/_internal/pub_generated/lib/src/barback/cycle_exception.dart
deleted file mode 100644
index 8fdd04788b4df97719f2c15708fda8b6dd3951e9..0000000000000000000000000000000000000000
--- a/sdk/lib/_internal/pub_generated/lib/src/barback/cycle_exception.dart
+++ /dev/null
@@ -1,60 +0,0 @@
-// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS d.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.barback.cycle_exception;
-
-import '../exceptions.dart';
-
-/// An exception thrown when a transformer dependency cycle is detected.
-///
-/// A cycle exception is usually produced within a deeply-nested series of
-/// calls. The API is designed to make it easy for each of these calls to add to
-/// the message so that the full reasoning for the cycle is made visible to the
-/// user.
-///
-/// Each call's individual message is called a "step". A [CycleException] is
-/// represented internally as a linked list of steps.
-class CycleException implements ApplicationException {
-  /// The step for this exception.
-  final String _step;
-
-  /// The next exception in the linked list.
-  ///
-  /// [_next]'s steps come after [_step].
-  final CycleException _next;
-
-  /// A list of all steps in the cycle.
-  List<String> get steps {
-    if (_step == null) return [];
-
-    var exception = this;
-    var steps = [];
-    while (exception != null) {
-      steps.add(exception._step);
-      exception = exception._next;
-    }
-    return steps;
-  }
-
-  String get message {
-    var steps = this.steps;
-    if (steps.isEmpty) return "Transformer cycle detected.";
-    return "Transformer cycle detected:\n" +
-        steps.map((step) => "  $step").join("\n");
-  }
-
-  /// Creates a new [CycleException] with zero or one steps.
-  CycleException([this._step])
-      : _next = null;
-
-  CycleException._(this._step, this._next);
-
-  /// Returns a copy of [this] with [step] added to the beginning of [steps].
-  CycleException prependStep(String step) {
-    if (_step == null) return new CycleException(step);
-    return new CycleException._(step, this);
-  }
-
-  String toString() => message;
-}