Add isWithin to pkg/path.

pkg/path/test/posix_test.dart

Index: pkg/path/test/posix_test.dart
diff --git a/pkg/path/test/posix_test.dart b/pkg/path/test/posix_test.dart
index 1156379a8555dc574f839c8523fdeb277f4baa0d..5bb38d0321907ff6a0581d6e588c8f166a12d31e 100644
--- a/pkg/path/test/posix_test.dart
+++ b/pkg/path/test/posix_test.dart
@@ -411,6 +411,27 @@ main() {
     });
   });
 
+  group('isWithin', () {
+    test('simple cases', () {
+      expect(builder.isWithin('foo/bar', 'foo/bar'), isFalse);
+      expect(builder.isWithin('foo/bar', 'foo/bar/baz'), isTrue);
+      expect(builder.isWithin('foo/bar', 'foo/baz'), isFalse);
+      expect(builder.isWithin('foo/bar', '../path/foo/bar/baz'), isTrue);
+      expect(builder.isWithin('/', '/foo/bar'), isTrue);
+      expect(builder.isWithin('baz', '/root/path/baz/bang'), isTrue);
+      expect(builder.isWithin('baz', '/root/path/bang/baz'), isFalse);
+    });
+
+    test('from a relative root', () {
+      var r = new path.Builder(style: path.Style.posix, root: 'foo/bar');
+      expect(builder.isWithin('.', 'a/b/c'), isTrue);
+      expect(builder.isWithin('.', '../a/b/c'), isFalse);
+      expect(builder.isWithin('.', '../../a/foo/b/c'), isFalse);
+      expect(builder.isWithin('/', '/baz/bang'), isTrue);
+      expect(builder.isWithin('.', '/baz/bang'), isFalse);
+    });
+  });
+
   group('resolve', () {
     test('allows up to seven parts', () {
       expect(builder.resolve('a'), '/root/path/a');