Adding more relative path support.

tests/standalone/io/path_test.dart

Index: tests/standalone/io/path_test.dart
diff --git a/tests/standalone/io/path_test.dart b/tests/standalone/io/path_test.dart
index b448c9a379ed0fc54dd4854d65ff15df42a97a7b..a2b39a603dbbdbb3b5baf3a200e31d5b1ca87791 100644
--- a/tests/standalone/io/path_test.dart
+++ b/tests/standalone/io/path_test.dart
@@ -10,6 +10,7 @@ void main() {
   testBaseFunctions();
   testCanonicalize();
   testJoinAppend();
+  testRelativeTo();
 }
 
 void testBaseFunctions() {
@@ -163,8 +164,9 @@ void testJoinAppend() {
   // .join can only join a relative path to a path.
   // It cannot join an absolute path to a path.
   Expect.throws(() => new Path('/a/b/').join(new Path('/c/d')));
+}
 
-  // Test Path.relativeTo.
+void testRelativeTo() {
   Expect.equals('c/d',
                 new Path('/a/b/c/d').relativeTo(new Path('/a/b')).toString());
   Expect.equals('c/d',
@@ -172,6 +174,19 @@ void testJoinAppend() {
   Expect.equals('.',
                 new Path('/a').relativeTo(new Path('/a')).toString());
 
+  // Trailing / in base path represents directory
+  Expect.equals('../../z/x/y',
+      new Path('/a/b/z/x/y').relativeTo(new Path('/a/b/c/d/')).toString());
+  Expect.equals('../z/x/y',
+      new Path('/a/b/z/x/y').relativeTo(new Path('/a/b/c/d')).toString());
+  Expect.equals('../z/x/y/',
+      new Path('/a/b/z/x/y/').relativeTo(new Path('/a/b/c/d')).toString());
+
+  Expect.equals('../../z/x/y',
+      new Path('/z/x/y').relativeTo(new Path('/a/b/c')).toString());
+  Expect.equals('../../../z/x/y',
+      new Path('/z/x/y').relativeTo(new Path('/a/b/c/')).toString());
+
   // Not implemented yet.  Should return new Path('../b/c/d/').
   Expect.throws(() =>
                 new Path('b/c/d/').relativeTo(new Path('a/')));