Create an apptesting framework for dart.

third_party/dart-packages/path/path/src/context.dart

Index: third_party/dart-packages/path/path/src/context.dart
diff --git a/sky/tests/resources/third_party/path/src/context.dart b/third_party/dart-packages/path/path/src/context.dart
similarity index 98%
copy from sky/tests/resources/third_party/path/src/context.dart
copy to third_party/dart-packages/path/path/src/context.dart
index a3efd95a9cbd4c4b5829df32149857911d4f1028..c2fe48d0d17b618ff43702302659786ea75a1aee 100644
--- a/sky/tests/resources/third_party/path/src/context.dart
+++ b/third_party/dart-packages/path/path/src/context.dart
@@ -324,7 +324,13 @@ class Context {
   /// "/", no path can be determined. In this case, a [PathException] will be
   /// thrown.
   String relative(String path, {String from}) {
-    from = from == null ? current : this.join(current, from);
+    // Avoid calling [current] since it is slow and calling join() when
+    // [from] is absolute does nothing.
+    if (from == null) {
+      from = current;
+    } else if (this.isRelative(from) || this.isRootRelative(from)) {
+      from = this.join(current, from);
+    }
 
     // We can't determine the path from a relative path to an absolute path.
     if (this.isRelative(from) && this.isAbsolute(path)) {