Fix a crash in pub's version solver.

sdk/lib/_internal/pub/lib/src/package.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.package;
 
 import 'dart:io';
 
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as p;
@@ skipping 381 matching lines @@
 }
 
 /// A reference to a constrained range of versions of one package.
 class PackageDep extends _PackageName {
   /// The allowed package versions.
   final VersionConstraint constraint;
 
   PackageDep(String name, String source, this.constraint, description)
       : super(name, source, description);
 
+  PackageDep.magic(String name)
+      : super.magic(name),
+        constraint = Version.none;
+
   String toString() {
     if (isRoot) return "$name $constraint (root)";
+    if (isMagic) return name;
     return "$name $constraint from $source ($description)";
   }
 
   int get hashCode => name.hashCode ^ source.hashCode;
 
   bool operator ==(other) {
     // TODO(rnystrom): We're assuming here that we don't need to delve into the
     // description.
     return other is PackageDep &&
            other.name == name &&
            other.source == source &&
            other.constraint == constraint;
   }
 }