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 {

[nweiz, 2013/03/29 01:58:25]

I don't understand why this change is in this CL.

[Bob Nystrom, 2013/03/30 00:15:55]

The previous code will fail if you call descriptionsEqual() and either path
doesn't exist. We have tests specifically for working with path deps to
non-existent paths. I believe they passed coincidentally because the old solver
happened to not hit this before it failed to find a solution, but the new solver
does end up comparing descriptions and would crash.

I think this change is a decent one: I don't think descriptionsEqual() should
ever throw given valid arguments.

+      // 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

[nweiz, 2013/03/29 01:58:25]

"fallback" -> "fall back"

[Bob Nystrom, 2013/03/30 00:15:55]

Done.

+      // the normalized paths.
+      var path1 = path.normalize(description1["path"]);
+      var path2 = path.normalize(description2["path"]);

[nweiz, 2013/03/29 01:58:25]

Shouldn't you make these absolute here?

[Bob Nystrom, 2013/03/30 00:15:55]

Done.

+      return path1 == path2;
+    }
   }
 
   Future<bool> install(PackageId id, String destination) {