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

utils/pub/sdk_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('sdk_source');
 
 #import('io.dart');
 #import('package.dart');
 #import('source.dart');
 
 /**
  * A package source that uses libraries from the Dart SDK.
  *
  * This currently uses the "sdkdir" command-line argument to find the SDK.
  */
 // TODO(nweiz): This should read the SDK directory from an environment variable
 // once we can set those for tests.
 class SdkSource extends Source {
   final String name = "sdk";
   final bool shouldCache = false;
 
   /**
    * The root directory of the Dart SDK.
    */
   final String rootDir;
 
   SdkSource(this.rootDir);
 
   /**
+   * An SDK package has no dependencies. Its version number is inferred from the
+   * revision number of the SDK itself.
+   */
+  Future<Pubspec> describe(PackageId id) {
+    return readTextFile(join(rootDir, "revision")).transform((revision) =>
+        new Pubspec("0.0.0-r.${revision.trim()}", <PackageRef>[]));
+  }
+
+  /**
    * Since all the SDK files are already available locally, installation just
    * involves symlinking the SDK library into the packages directory.
    */
   Future<bool> install(PackageId id, String destPath) {
     var sourcePath = join(rootDir, "lib", id.description);
     return exists(sourcePath).chain((exists) {
       if (!exists) return new Future<bool>.immediate(false);
       return createSymlink(sourcePath, destPath).transform((_) => true);
     });
   }
 }