Add a validator for dependency version 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 'io.dart';
 import 'lock_file.dart';
 import 'log.dart' as log;
 import 'package.dart';
@@ skipping 96 matching lines @@
 
     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 _loadLockFile()
+    return loadLockFile()
       .chain((lockFile) => resolveVersions(cache.sources, root, lockFile))
       .chain(_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())
       .chain(_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 _loadLockFile().chain((lockFile) {
+    return loadLockFile().chain((lockFile) {
       var versionSolver = new VersionSolver(cache.sources, root, lockFile);
       for (var dependency in dependencies) {
         versionSolver.useLatestVersion(dependency);
       }
       return versionSolver.solve();
     }).chain(_installDependencies);
   }
 
   /**
    * Removes the old packages directory, installs all dependencies listed in
    * [packageVersions], and writes a [LockFile].
    */
   Future _installDependencies(List<PackageId> packageVersions) {
     return cleanDir(path).chain((_) {
       return Futures.wait(packageVersions.map((id) {
         if (id.source is RootSource) return new Future.immediate(id);
         return install(id);
       }));
     }).chain(_saveLockFile)
       .chain(_installSelfReference)
       .chain(_linkSecondaryPackageDirs);
   }
 
-  /**
-   * Loads the list of concrete package versions from the `pubspec.lock`, if it
-   * exists. If it doesn't, this completes to an empty [LockFile].
-   *
-   * If there's an error reading the `pubspec.lock` file, this will print a
-   * warning message and act as though the file doesn't exist.
-   */
-  Future<LockFile> _loadLockFile() {
+  /// Loads the list of concrete package versions from the `pubspec.lock`, if it
+  /// exists. If it doesn't, this completes to an empty [LockFile].
+  Future<LockFile> loadLockFile() {
     var lockFilePath = join(root.dir, 'pubspec.lock');
 
     log.fine("Loading lockfile.");
     return fileExists(lockFilePath).chain((exists) {
       if (!exists) {
         log.fine("No lock file at $lockFilePath, creating empty one.");
         return new Future<LockFile>.immediate(new LockFile.empty());
       }
 
       return readTextFile(lockFilePath).transform((text) =>
@@ skipping 95 matching lines @@
    * Creates a symlink to the `packages` directory in [dir] if none exists.
    */
   Future _linkSecondaryPackageDir(String dir) {
     var to = join(dir, 'packages');
     return exists(to).chain((exists) {
       if (exists) return new Future.immediate(null);
       return createSymlink(path, to);
     });
   }
 }