Move some core classes to collection library.

utils/pub/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.
 
 /// Attempts to resolve a set of version constraints for a package dependency
 /// graph and select an appropriate set of best specific versions for all
 /// dependent packages. It works iteratively and tries to reach a stable
 /// solution where the constraints of all dependencies are met. If it fails to
 /// reach a solution after a certain number of iterations, it assumes the
 /// dependency graph is unstable and reports and error.
@@ skipping 18 matching lines @@
 /// constraint on that package. I.e. with a shared dependency, we intersect all
 /// of the constraints that its depending packages place on it. If that overall
 /// constraint changes (say from "<3.0.0" to "<2.5.0"), then the currently
 /// picked version for that package may fall outside of the new constraint. If
 /// that happens, we find the new best version that meets the updated constraint
 /// and then the change the package to use that version. That cycles back up to
 /// the beginning again.
 library version_solver;
 
 import 'dart:async';
+import 'dart:collection' show Queue;
 import 'dart:json' as json;
 import 'dart:math';
 import 'lock_file.dart';
 import 'log.dart' as log;
 import 'package.dart';
 import 'pubspec.dart';
 import 'root_source.dart';
 import 'source.dart';
 import 'source_registry.dart';
 import 'utils.dart';
@@ skipping 651 matching lines @@
 
   String toString() {
     // TODO(nweiz): Dump descriptions to YAML when that's supported.
     return "Incompatible dependencies on '$package':\n"
         "- '$depender1' depends on it with description "
         "${json.stringify(description1)}\n"
         "- '$depender2' depends on it with description "
         "${json.stringify(description2)}";
   }
 }