Allow `pub install` and `pub update` to update dependers.

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('io.dart');
 #import('lock_file.dart');
 #import('package.dart');
 #import('root_source.dart');
@@ skipping 105 matching lines @@
    * 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()).
       chain(_installDependencies);
   }
 
   /**
    * Installs the latest available versions of [dependencies], while leaving
-   * other dependencies as specified by the [LockFile]. Returns a [Future] that
-   * completes when all dependencies are installed.
+   * 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 _loadLockFile().chain((lockFile) {
+      var versionSolver = new VersionSolver(cache.sources, root, lockFile);
       for (var dependency in dependencies) {
-        // TODO(nweiz): How do we want to detect and handle unknown
-        // dependencies here?
-        lockFile.packages.remove(dependency);
+        versionSolver.useBestVersion(dependency);
       }
-      return resolveVersions(cache.sources, root, lockFile);
+      return versionSolver.solve();
     }).chain(_installDependencies);
   }
 
   /**
    * Installs all dependencies listed in [packageVersions] and writes a
    * [LockFile].
    */
   Future _installDependencies(List<PackageId> packageVersions) {
     return Futures.wait(packageVersions.map((id) {
       if (id.source is RootSource) return new Future.immediate(id);
@@ skipping 36 matching lines @@
    */
   Future _saveLockFile(List<PackageId> packageIds) {
     var lockFile = new LockFile.empty();
     for (var id in packageIds) {
       if (id.source is! RootSource) lockFile.packages[id.name] = id;
     }
 
     return writeTextFile(join(root.dir, 'pubspec.lock'), lockFile.serialize());
   }
 }