Add toUri and fromUri functions to pathos.

pkg/pathos/test/pathos_posix_test.dart

Index: pkg/pathos/test/pathos_posix_test.dart
diff --git a/pkg/pathos/test/pathos_posix_test.dart b/pkg/pathos/test/pathos_posix_test.dart
index ecdd1336ba3c39eac02bc4fda3edb9d663cf0121..d0b3b94b749af9f441fce07859d30480485db6b7 100644
--- a/pkg/pathos/test/pathos_posix_test.dart
+++ b/pkg/pathos/test/pathos_posix_test.dart
@@ -399,4 +399,20 @@ main() {
     expect(builder.withoutExtension('a/b.c/'), 'a/b/');
     expect(builder.withoutExtension('a/b.c//'), 'a/b//');
   });
+
+  test('fromUri', () {
+    expect(builder.fromUri(Uri.parse('file:///path/to/foo')), '/path/to/foo');
+    expect(builder.fromUri(Uri.parse('file:///')), '/');
+    expect(builder.fromUri(Uri.parse('foo/bar')), 'foo/bar');
+    expect(builder.fromUri(Uri.parse('/path/to/foo')), '/path/to/foo');
+    expect(builder.fromUri(Uri.parse('///path/to/foo')), '/path/to/foo');
+    expect(() => builder.fromUri(Uri.parse('http://dartlang.org')),
+        throwsArgumentError);
+  });
+
+  test('toUri', () {
+    expect(builder.toUri('/path/to/foo'), Uri.parse('file:///path/to/foo'));
+    expect(builder.toUri('/'), Uri.parse('file:///'));
+    expect(builder.toUri('foo/bar'), Uri.parse('foo/bar'));

[Bob Nystrom, 2013/06/13 00:21:18]

This feels odd to me. Will users expect this, or should we convert "foo/bar" to
an absolute path and then return a "file:" URL?

[nweiz, 2013/06/19 00:50:27]

I think this is correct. Think about it from the perspective of manipulating
paths and imported URIs. If you know the relative path from file A to file B and
you want to convert that to an importable URI, you definitely want it to stay
relative during that conversion.

+  });
 }