Prefer stable versions over pre-release versions in the solver.

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';
@@ skipping 103 matching lines @@
     return source.describe(id).then((pubspec) {
       _pubspecs[id] = pubspec;
       return pubspec;
     });
   }
 
   /// Returns the previously cached pubspec for the package identified by [id]
   /// or returns `null` if not in the cache.
   Pubspec getCachedPubspec(PackageId id) => _pubspecs[id];
 
-  /// Gets the list of versions for [package] in descending order.
+  /// Gets the list of versions for [package].
+  ///
+  /// Packages are sorted in descending version order with all "stable"
+  /// versions (i.e. ones without a prerelease suffix) before pre-release
+  /// versions. This ensures that the solver prefers stable packages over
+  /// unstable ones.
   Future<List<PackageId>> getVersions(PackageRef package) {
     if (package.isRoot) {
       throw new StateError("Cannot get versions for root package $package.");
     }
 
     // See if we have it cached.
     var versions = _versions[package];
     if (versions != null) {
       versionCacheHits++;
       return new Future.value(versions);
     }
 
     versionCacheMisses++;
 
     var source = _sources[package.source];
     return source.getVersions(package.name, package.description)
         .then((versions) {
       // Sort by descending version so we try newer versions first.
-      versions.sort((a, b) => b.compareTo(a));
+      versions.sort((a, b) {
+        // Sort all prerelease versions after all normal versions. This way
+        // the solver will prefer stable packages over unstable ones.
+        if (a.isPreRelease && !b.isPreRelease) return 1;
+        if (!a.isPreRelease && b.isPreRelease) return -1;
+
+        return b.compareTo(a);
+      });
 
       var ids = versions.map((version) => package.atVersion(version)).toList();
       _versions[package] = ids;
       return ids;
     });
   }
 }
 
 /// A reference from a depending package to a package that it depends on.
 class Dependency {
@@ skipping 136 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)}";
   }
 }