Use backtracking when solving dependency constraints.

utils/pub/path_source.dart

Index: utils/pub/path_source.dart
diff --git a/utils/pub/path_source.dart b/utils/pub/path_source.dart
index 6cdc0707bbb76add33eaf057f917a809446c3060..fc7ff93faa5efac05c12c628a9ac168e91fe06ef 100644
--- a/utils/pub/path_source.dart
+++ b/utils/pub/path_source.dart
@@ -33,10 +33,18 @@ class PathSource extends Source {
   }
 
   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, fallback to just comparing
+      // the normalized paths.
+      var path1 = path.normalize(description1["path"]);
+      var path2 = path.normalize(description2["path"]);
+      return path1 == path2;
+    }
   }
 
   Future<bool> install(PackageId id, String destination) {