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

sdk/lib/_internal/pub_generated/lib/src/solver/version_queue.dart

Index: sdk/lib/_internal/pub_generated/lib/src/solver/version_queue.dart
diff --git a/sdk/lib/_internal/pub_generated/lib/src/solver/version_queue.dart b/sdk/lib/_internal/pub_generated/lib/src/solver/version_queue.dart
deleted file mode 100644
index 86c6e987128395801cfe1d40bcadb6acfde525eb..0000000000000000000000000000000000000000
--- a/sdk/lib/_internal/pub_generated/lib/src/solver/version_queue.dart
+++ /dev/null
@@ -1,110 +0,0 @@
-// Copyright (c) 2013, 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.solver.version_queue;
-
-import 'dart:async';
-import 'dart:collection' show Queue;
-
-import '../package.dart';
-
-/// A function that asynchronously returns a sequence of package IDs.
-typedef Future<Iterable<PackageId>> PackageIdGenerator();
-
-/// A prioritized, asynchronous queue of the possible versions that can be
-/// selected for one package.
-///
-/// If there is a locked version, that comes first, followed by other versions
-/// in descending order. This avoids requesting the list of versions until
-/// needed (i.e. after any locked version has been consumed) to avoid unneeded
-/// network requests.
-class VersionQueue {
-  /// The set of allowed versions that match [_constraint].
-  ///
-  /// If [_locked] is not `null`, this will initially be `null` until we
-  /// advance past the locked version.
-  Queue<PackageId> _allowed;
-
-  /// The callback that will generate the sequence of packages. This will be
-  /// called as lazily as possible.
-  final PackageIdGenerator _allowedGenerator;
-
-  /// The currently locked version of the package, or `null` if there is none,
-  /// or we have advanced past it.
-  PackageId _locked;
-
-  /// Gets the currently selected version.
-  PackageId get current {
-    if (_locked != null) return _locked;
-    return _allowed.first;
-  }
-
-  /// Whether the currently selected version has been responsible for a solve
-  /// failure, or depends on a package that has.
-  ///
-  /// The solver uses this to determine which packages to backtrack to after a
-  /// failure occurs. Any selected package that did *not* cause the failure can
-  /// be skipped by the backtracker.
-  bool get hasFailed => _hasFailed;
-  bool _hasFailed = false;
-
-  /// Creates a new [VersionQueue] queue for starting with the optional
-  /// [locked] package followed by the results of calling [allowedGenerator].
-  ///
-  /// This is asynchronous so that [current] can always be accessed
-  /// synchronously. If there is no locked version, we need to get the list of
-  /// versions asynchronously before we can determine what the first one is.
-  static Future<VersionQueue> create(PackageId locked,
-      PackageIdGenerator allowedGenerator) {
-    var versions = new VersionQueue._(locked, allowedGenerator);
-
-    // If there is a locked version, it's the current one so it's synchronously
-    // available now.
-    if (locked != null) return new Future.value(versions);
-
-    // Otherwise, the current version needs to be calculated before we can
-    // return.
-    return versions._calculateAllowed().then((_) => versions);
-  }
-
-  VersionQueue._(this._locked, this._allowedGenerator);
-
-  /// Tries to advance to the next possible version.
-  ///
-  /// Returns `true` if it moved to a new version (which can be accessed from
-  /// [current]. Returns `false` if there are no more versions.
-  Future<bool> advance() {
-    // Any failure was the fault of the previous version, not necessarily the
-    // new one.
-    _hasFailed = false;
-
-    // If we have a locked version, consume it first.
-    if (_locked != null) {
-      // Advancing past the locked version, so need to load the others now
-      // so that [current] is available.
-      return _calculateAllowed().then((_) {
-        _locked = null;
-        return _allowed.isNotEmpty;
-      });
-    }
-
-    // Move to the next allowed version.
-    _allowed.removeFirst();
-    return new Future.value(_allowed.isNotEmpty);
-  }
-
-  /// Marks the selected version as being directly or indirectly responsible
-  /// for a solve failure.
-  void fail() {
-    _hasFailed = true;
-  }
-
-  /// Determines the list of allowed versions matching its constraint and places
-  /// them in [_allowed].
-  Future _calculateAllowed() {
-    return _allowedGenerator().then((allowed) {
-      _allowed = new Queue<PackageId>.from(allowed);
-    });
-  }
-}