Add support to the version solver for sources that only have a single version per package.

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('io.dart');
 #import('package.dart');
 #import('pubspec.dart');
 #import('system_cache.dart');
@@ skipping 53 matching lines @@
     this._systemCache = systemCache;
   }
 
   /**
    * Get the list of all versions that exist for the package described by
    * [description].
    *
    * 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).
+   *
+   * By default, this assumes that each description has a single version and
+   * uses [describe] to get that version.
    */
   Future<List<Version>> getVersions(description) {
-    // TODO(rnystrom): Do something better here.
-    throw "Source $name doesn't support versioning.";
+    return describe(new PackageId(this, Version.none, description))
+      .transform((pubspec) => [pubspec.version]);
   }
 
   /**
    * Loads the (possibly remote) pubspec for the package version identified by
-   * [id]. This will be called for packages that have not yet been installed
+   * [id]. This may be called for packages that have not yet been installed
    * during the version resolution process.
+   *
+   * For cached sources, by default this uses [installToSystemCache] to get the
+   * pubspec. There is no default implementation for non-cached sources; they
+   * must implement it manually.
    */
   Future<Pubspec> describe(PackageId id) {
-    // TODO(rnystrom): Figure out how non-default sources should handle this.
-    throw "Source $name doesn't support versioning.";
+    if (!shouldCache) throw "Source $name must implement describe(id).";
+    return installToSystemCache(id).transform((package) => package.pubspec);
   }
 
   /**
    * 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.
@@ skipping 60 matching lines @@
    * Returns whether or not [description1] describes the same package as
    * [description2] for this source. This method should be light-weight. It
    * doesn't need to validate that either package exists.
    *
    * By default, this assumes both descriptions are strings and compares them
    * for equality.
    */
   bool descriptionsEqual(description1, description2) =>
     description1 == description2;
 }