Rewrite pub's version solver.

sdk/lib/_internal/pub/lib/src/solver/dependency_queue.dart

Index: sdk/lib/_internal/pub/lib/src/solver/dependency_queue.dart
diff --git a/sdk/lib/_internal/pub/lib/src/solver/dependency_queue.dart b/sdk/lib/_internal/pub/lib/src/solver/dependency_queue.dart
deleted file mode 100644
index e802154fe37e1571897fd821c73e79011ceb4178..0000000000000000000000000000000000000000
--- a/sdk/lib/_internal/pub/lib/src/solver/dependency_queue.dart
+++ /dev/null
@@ -1,150 +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.dependency_queue;
-
-import 'dart:async';
-import 'dart:collection' show Queue;
-
-import '../log.dart' as log;
-import '../package.dart';
-import 'backtracking_solver.dart';
-
-/// A queue of one package's dependencies, ordered by how the solver should
-/// traverse them.
-///
-/// It prefers locked versions so that they stay locked if possible. Then it
-/// prefers a currently selected package so that it only has to consider a
-/// single version.
-///
-/// After that, it orders the remaining packages by the number of versions they
-/// have so that packages with fewer versions are solved first. (If two
-/// packages have the same number of versions, they are sorted alphabetically
-/// just to be deterministic.)
-///
-/// Critically, this queue will *not* sort the dependencies by number of
-/// versions until actually needed. This ensures we don't do any network
-/// requests until we actually need to. In particular, it means that solving
-/// a package graph with an already up-to-date lockfile will do no network
-/// requests.
-class DependencyQueue {
-  final BacktrackingSolver _solver;
-
-  /// The dependencies for packages that have already been selected.
-  final Queue<PackageDep> _presorted;
-
-  /// The dependencies on the remaining packages.
-  ///
-  /// This is lazily sorted right before the first item is requested.
-  final List<PackageDep> _remaining;
-
-  bool _isSorted = false;
-
-  /// Gets whether there are any dependencies left to iterate over.
-  bool get isEmpty => _presorted.isEmpty && _remaining.isEmpty;
-
-  /// The pending [Future] while the remaining dependencies are being sorted.
-  ///
-  /// This will only be non-null while a sort is in progress.
-  Future _sortFuture;
-
-  factory DependencyQueue(BacktrackingSolver solver,
-      Iterable<PackageDep> deps) {
-    // Separate out the presorted ones.
-    var presorted = <PackageDep>[];
-    var remaining = <PackageDep>[];
-
-    for (var dep in deps) {
-      // Selected or locked packages come first.
-      if (solver.getSelected(dep.name) != null ||
-          solver.getLocked(dep.name) != null) {
-        presorted.add(dep);
-      } else {
-        remaining.add(dep);
-      }
-    }
-
-    // Sort the selected/locked packages by name just to ensure the solver is
-    // deterministic.
-    presorted.sort((a, b) => a.name.compareTo(b.name));
-
-    return new DependencyQueue._(solver, new Queue<PackageDep>.from(presorted),
-        remaining);
-  }
-
-  DependencyQueue._(this._solver, this._presorted, this._remaining);
-
-  /// Emits the next dependency in priority order.
-  ///
-  /// It is an error to call this if [isEmpty] returns `true`. Note that this
-  /// function is *not* re-entrant. You should only advance after the previous
-  /// advance has completed.
-  Future<PackageDep> advance() async {
-    // Emit the sorted ones first.
-    if (_presorted.isNotEmpty) return _presorted.removeFirst();
-
-    // Sort the remaining packages when we need the first one.
-    if (!_isSorted) await _sort();
-    return _remaining.removeAt(0);
-  }
-
-  /// Sorts the unselected packages by number of versions and name.
-  Future _sort() {
-    // Sorting is not re-entrant.
-    assert(_sortFuture == null);
-
-    _sortFuture = Future.wait(_remaining.map(_getNumVersions)).then((versions) {
-      _sortFuture = null;
-
-      // Map deps to the number of versions they have.
-      var versionMap = new Map.fromIterables(_remaining, versions);
-
-      // Sort in best-first order to minimize backtracking.
-      _remaining.sort((a, b) {
-        // Traverse into packages with fewer versions since they will lead to
-        // less backtracking.
-        if (versionMap[a] != versionMap[b]) {
-          return versionMap[a].compareTo(versionMap[b]);
-        }
-
-        // Otherwise, just sort by name so that it's deterministic.
-        return a.name.compareTo(b.name);
-      });
-
-      _isSorted = true;
-    });
-
-    return _sortFuture;
-  }
-
-  /// Given a dependency, returns a future that completes to the number of
-  /// versions available for it.
-  Future<int> _getNumVersions(PackageDep dep) async {
-    // There is only ever one version of the root package.
-    if (dep.isRoot) return 1;
-
-    var versions;
-    try {
-      versions = await _solver.cache.getVersions(dep.toRef());
-    } catch (error, stackTrace) {
-      // If it fails for any reason, just treat that as no versions. This
-      // will sort this reference higher so that we can traverse into it
-      // and report the error more properly.
-      log.solver("Could not get versions for $dep:\n$error\n\n$stackTrace");
-      return 0;
-    }
-
-    // If the root package depends on this one, ignore versions that don't match
-    // that constraint. Since the root package's dependency constraints won't
-    // change during solving, we can safely filter out packages that don't meet
-    // it.
-    for (var rootDep in _solver.root.immediateDependencies) {
-      if (rootDep.name != dep.name) continue;
-      return versions.where((id) => rootDep.constraint.allows(id.version))
-          .length;
-    }
-
-    return versions.length;
-  }
-}