Make VersionSolver source- and description-aware.

utils/pub/root_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('root_source');
+
+#import('package.dart');
+#import('source.dart');
+
+/**
+ * A source used only for the root package when doing version resolution. It
+ * contains only the root package and is unable to install packages.
+ *
+ * This source cannot be referenced from a pubspec.
+ */
+class RootSource extends Source {
+  final String name = "root";
+  final bool shouldCache = false;
+  final Package package;
+
+  RootSource(this.package);
+
+  Future<List<Version>> getVersions(description) {
+    return Future<List<Version>>.immediate([package.version]);
+  }
+
+  Future<Pubspec> describe(PackageId id) {
+    return Future<Pubspec>.immediate(package.pubspec);
+  }
+
+  Future<bool> install(PackageId id, String destPath) {
+    throw new UnsupportedOperationException(
+        "Can't install from a root source.");
+  }
+}