Disable backtracking solver.

utils/pub/solver/version_solver.dart

 // Copyright (c) 2012, 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 version_solver;
 
 import 'dart:async';
 import 'dart:json' as json;
 
 import '../lock_file.dart';
 import '../log.dart' as log;
 import '../package.dart';
 import '../pubspec.dart';
 import '../source.dart';
 import '../source_registry.dart';
 import '../version.dart';
 import 'backtracking_solver.dart';
 import 'greedy_solver.dart';
 
 /// Attempts to select the best concrete versions for all of the transitive
 /// dependencies of [root] taking into account all of the [VersionConstraint]s
 /// that those dependencies place on each other and the requirements imposed by
 /// [lockFile].
 ///
 /// If [useLatest] is given, then only the latest versions of the referenced
 /// packages will be used. This is for forcing an update to one or more
 /// packages.
 ///
-/// If [allowBacktracking] is `false` the non-backtracking version solver will
-/// be used. Otherwise, the backtracking one will be.
+/// If [allowBacktracking] is `true` the backtracking version solver will
+/// be used. Otherwise, the non-backtracking one will be.
 Future<SolveResult> resolveVersions(SourceRegistry sources, Package root,
     {LockFile lockFile, bool allowBacktracking, List<PackageRef> useLatest}) {
   log.message('Resolving dependencies...');
 
-  if (allowBacktracking == null) allowBacktracking = true;
+  if (allowBacktracking == null) allowBacktracking = false;
   if (lockFile == null) lockFile = new LockFile.empty();
   if (useLatest == null) useLatest = [];
 
   var solver;
   if (allowBacktracking) {
     solver = new BacktrackingVersionSolver(sources, root, lockFile, useLatest);
   } else {
     solver = new GreedyVersionSolver(sources, root, lockFile, useLatest);
   }
 
@@ skipping 324 matching lines @@
       Iterable<Dependency> dependencies)
       : super(package, dependencies);
 
   String get _message => "Incompatible dependencies on '$package'";
 
   String _describeDependency(PackageRef ref) {
     // TODO(nweiz): Dump descriptions to YAML when that's supported.
     return "depends on it with description ${json.stringify(ref.description)}";
   }
 }