Use backtracking when solving dependency constraints.

utils/pub/path_source.dart

 // Copyright (c) 2013, 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 path_source;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:pathos/path.dart' as path;
@@ skipping 15 matching lines @@
 
   Future<Pubspec> describe(PackageId id) {
     return defer(() {
       _validatePath(id.name, id.description);
       return new Pubspec.load(id.name, id.description["path"],
           systemCache.sources);
     });
   }
 
   bool descriptionsEqual(description1, description2) {
-    // Compare real paths after normalizing and resolving symlinks.
-    var path1 = new File(description1["path"]).fullPathSync();
-    var path2 = new File(description2["path"]).fullPathSync();
-    return path1 == path2;
+    try {
+      // Compare real paths after normalizing and resolving symlinks.
+      var path1 = new File(description1["path"]).fullPathSync();
+      var path2 = new File(description2["path"]).fullPathSync();
+      return path1 == path2;
+    } on FileIOException catch (ex) {
+      // If either of the files couldn't be found, fall back to just comparing
+      // the normalized paths.
+      var path1 = path.normalize(path.absolute(description1["path"]));
+      var path2 = path.normalize(path.absolute(description2["path"]));
+      return path1 == path2;
+    }
   }
 
   Future<bool> install(PackageId id, String destination) {
     return defer(() {
       try {
         _validatePath(id.name, id.description);
       } on FormatException catch(err) {
         return false;
       }
 
@@ skipping 60 matching lines @@
 
     if (fileExists(dir)) {
       throw new FormatException(
           "Path dependency for package '$name' must refer to a "
           "directory, not a file. Was '$dir'.");
     }
 
     throw new FormatException("Could not find package '$name' at '$dir'.");
   }
 }