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

utils/pub/sdk_source.dart

Index: utils/pub/sdk_source.dart
diff --git a/utils/pub/sdk_source.dart b/utils/pub/sdk_source.dart
index ef79fdcf690c11622ecc45afb40e2d2947402859..3662f853fa4530280307cbfe9e5b2369fb43aa6c 100644
--- a/utils/pub/sdk_source.dart
+++ b/utils/pub/sdk_source.dart
@@ -27,6 +27,15 @@ class SdkSource extends Source {
   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.
    */