Fix warnings.

sdk/lib/_internal/pub/lib/src/solver/version_solver.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 pub.solver.version_solver;
 
 import 'dart:async';
 import "dart:convert";
 
 import '../lock_file.dart';
 import '../log.dart' as log;
 import '../package.dart';
 import '../pubspec.dart';
-import '../source.dart';
 import '../source_registry.dart';
 import '../version.dart';
 import '../utils.dart';
 import 'backtracking_solver.dart';
 
 /// Attempts to select the best concrete versions for all of the transitive
 /// dependencies of [root] taking into account all of the [VersionConstraint]s
 /// that those dependencies place on each other and the requirements imposed by
 /// [lockFile].
 ///
@@ skipping 263 matching lines @@
       Iterable<Dependency> dependencies)
       : super(package, dependencies);
 
   String get _message => "Incompatible dependencies on '$package'";
 
   String _describeDependency(PackageDep dep) {
     // TODO(nweiz): Dump descriptions to YAML when that's supported.
     return "depends on it with description ${JSON.encode(dep.description)}";
   }
 }