Add isWithin to pkg/path.

pkg/path/test/relative_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 //
 // Test "relative" on all styles of path.Builder, on all platforms.
 
 import "package:unittest/unittest.dart";
 import "package:path/path.dart" as path;
 
+import "utils.dart";
+
 void main() {
   test("test relative", () {
     relativeTest(new path.Builder(style: path.Style.posix, root: '.'), '/');
     relativeTest(new path.Builder(style: path.Style.posix, root: '/'), '/');
     relativeTest(new path.Builder(style: path.Style.windows, root: r'd:\'),
                  r'c:\');
     relativeTest(new path.Builder(style: path.Style.windows, root: '.'),
                  r'c:\');
     relativeTest(new path.Builder(style: path.Style.url, root: 'file:///'),
                  'http://myserver/');
@@ skipping 48 matching lines @@
   expectRelative('a/b', 'x/y//a/b', 'x/y/');
   expectRelative('../ya/b', '${prefix}x/ya/b', '${prefix}x/y');
   expectRelative('../ya/b', 'x/ya/b', 'x/y');
   expectRelative('../b', 'x/y/../b', 'x/y/.');
   expectRelative('a/b/c', 'x/y/a//b/./f/../c', 'x/y');
   expectRelative('.', '${prefix}x/y//', '${prefix}x/y/');
   expectRelative('.', '${prefix}x/y/', '${prefix}x/y');
 
   // Should always throw - no relative path can be constructed.
   if (isRelative) {
-    expect(() => builder.relative('.', from: '..'), throwsArgumentError);
+    expect(() => builder.relative('.', from: '..'), throwsPathException);
     expect(() => builder.relative('a/b', from: '../../d'),
-           throwsArgumentError);
+           throwsPathException);
     expect(() => builder.relative('a/b', from: '${prefix}a/b'),
-           throwsArgumentError);
+           throwsPathException);
     // An absolute path relative from a relative path returns the absolute path.
     expectRelative('${prefix}a/b', '${prefix}a/b', 'c/d');
   }
 }