Use backtracking when solving dependency constraints.

utils/pub/entrypoint.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 entrypoint;
 
 import 'dart:async';
 
 import '../../pkg/pathos/lib/path.dart' as path;
 
@@ skipping 85 matching lines @@
     _installs[id] = future;
 
     return future;
   }
 
   /// Installs all dependencies of the [root] package to its "packages"
   /// directory, respecting the [LockFile] if present. Returns a [Future] that
   /// completes when all dependencies are installed.
   Future installDependencies() {
     return defer(() {
-      return resolveVersions(cache.sources, root, loadLockFile());
+      return resolveVersions(cache.sources, root, lockFile: loadLockFile());
     }).then(_installDependencies);
   }
 
   /// Installs the latest available versions of all dependencies of the [root]
   /// package to its "package" directory, writing a new [LockFile]. Returns a
   /// [Future] that completes when all dependencies are installed.
   Future updateAllDependencies() {
-    return resolveVersions(cache.sources, root, new LockFile.empty())
-        .then(_installDependencies);
+    return resolveVersions(cache.sources, root).then(_installDependencies);
   }
 
   /// Installs the latest available versions of [dependencies], while leaving
   /// other dependencies as specified by the [LockFile] if possible. Returns a
   /// [Future] that completes when all dependencies are installed.
   Future updateDependencies(List<String> dependencies) {
     return defer(() {
-      var solver = new VersionSolver(cache.sources, root, loadLockFile());
-      for (var dependency in dependencies) {
-        solver.useLatestVersion(dependency);
-      }
-      return solver.solve();
+      return resolveVersions(cache.sources, root,
+          lockFile: loadLockFile(), useLatest: dependencies);
     }).then(_installDependencies);
   }
 
   /// Removes the old packages directory, installs all dependencies listed in
   /// [packageVersions], and writes a [LockFile].
   Future _installDependencies(List<PackageId> packageVersions) {
     return new Future.of(() {
       cleanDir(packagesDir);
       return Future.wait(packageVersions.map((id) {
         if (id.isRoot) return new Future.immediate(id);
@@ skipping 158 matching lines @@
   /// Creates a symlink to the `packages` directory in [dir]. Will replace one
   /// if already there.
   Future _linkSecondaryPackageDir(String dir) {
     return defer(() {
       var symlink = path.join(dir, 'packages');
       if (entryExists(symlink)) deleteEntry(symlink);
       return createSymlink(packagesDir, symlink, relative: true);
     });
   }
 }