First pass at version constraint solver.

utils/pub/source.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('source');
 
 #import('package.dart');
+#import('pubspec.dart');
+#import('version.dart');
 
 /**
  * A source from which to install packages.
  *
  * Each source has many packages that it looks up using [PackageId]s. The source
  * is responsible for installing these packages to the package cache.
  */
 class Source {
   /**
    * The name of the source. Should be lower-case, suitable for use in a
    * filename, and unique accross all sources.
    */
   abstract String get name();
 
   /**
    * Whether this source's packages should be cached in Pub's global cache
    * directory.
    *
    * A source should be cached if it requires network access to retrieve
    * packages. It doesn't need to be cached if all packages are available
    * locally.
    */
   abstract bool get shouldCache();
 
   /**
+   * Get the list of all versions that exist for package [name].
+   *
+   * Note that this does *not* require the packages to be installed, which is
+   * the point. This is used during version resolution to determine which
+   * package versions are available to be installed (or already installed).
+   */
+  Future<List<Version>> getVersions(String name) {
+    // TODO(rnystrom): Do something better here.
+    throw "Source $name doesn't support versioning.";
+  }
+
+  /**
+   * Loads the (possibly remote) pubspec for the desired [version] of the named
+   * [package]. This will be called for packages that have not yet been
+   * installed during the version resolution process.
+   */
+  Future<Pubspec> describe(String package, Version version) {
+    // TODO(rnystrom): Figure out how non-default sources should handle this.
+    throw "Source $name doesn't support versioning.";
+  }
+
+  /**
    * Installs the package identified by [id] to [path]. Returns a [Future] that
    * completes when the installation was finished. The [Future] should resolve
    * to true if the package was found in the source and false if it wasn't. For
    * all other error conditions, it should complete with an exception.
    *
    * [path] is guaranteed not to exist, and its parent directory is guaranteed
    * to exist.
    */
   abstract Future<bool> install(PackageId id, String path);
 
@@ skipping 19 matching lines @@
   /**
    * Returns a human-friendly name for the package identified by [id]. This
    * method should be light-weight. It doesn't need to validate that the given
    * package exists.
    *
    * The package name should be lower-case and suitable for use in a filename.
    * It may contain forward slashes.
    */
   String packageName(PackageId id) => id.description;
 }