First pass at build dependency graph for barback.

pkg/pathos/lib/path.dart

Index: pkg/pathos/lib/path.dart
diff --git a/pkg/pathos/lib/path.dart b/pkg/pathos/lib/path.dart
index 80ffe72a57da05881db73e521e0753a0788d560b..b9c2936ecc2efb73a8d7188f4330cff5eee79768 100644
--- a/pkg/pathos/lib/path.dart
+++ b/pkg/pathos/lib/path.dart
@@ -203,6 +203,12 @@ String normalize(String path) => _builder.normalize(path);
 String relative(String path, {String from}) =>
     _builder.relative(path, from: from);
 
+/// Returns whether or not [path] is nested somewhere within [dir].
+///
+///     path.isWithin('a/b/c.dart', 'a/b') // -> true
+///     path.isWithin('a/b/c.dart', 'd/b') // -> false
+bool isWithin(String path, String dir) => _builder.isWithin(path, dir);
+
 /// Removes a trailing extension from the last part of [path].
 ///
 ///     withoutExtension('path/to/foo.dart'); // -> 'path/to/foo'
@@ -349,7 +355,16 @@ class Builder {
   /// On POSIX systems, absolute paths start with a `/` (forward slash). On
   /// Windows, an absolute path starts with `\\`, or a drive letter followed by
   /// `:/` or `:\`.
-  bool isRelative(String path) => !isAbsolute(path);
+  bool isRelative(String path) => !this.isAbsolute(path);
+
+  /// Returns whether or not [path] is nested somewhere within [dir].
+  ///
+  ///     path.isWithin('a/b/c.dart', 'a/b') // -> true
+  ///     path.isWithin('a/b/c.dart', 'd/b') // -> false
+  bool isWithin(String path, String dir) {
+    var relative = this.relative(path, from: dir);
+    return !this.isAbsolute(relative) && this.split(relative)[0] != '..';
+  }
 
   /// Joins the given path parts into a single path. Example:
   ///