Allow [path.fromUri] to take a string as well as a URI.

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 83a473e78087dc3119151529329c5ccab499b3fa..9da97c9660b86d7e44dc5c2e615da9eeee8aa3eb 100644
--- a/pkg/path/test/posix_test.dart
+++ b/pkg/path/test/posix_test.dart
@@ -474,19 +474,26 @@ main() {
     expect(context.withoutExtension('a/b.c//'), 'a/b//');
   });
 
-  test('fromUri', () {
-    expect(context.fromUri(Uri.parse('file:///path/to/foo')), '/path/to/foo');
-    expect(context.fromUri(Uri.parse('file:///path/to/foo/')), '/path/to/foo/');
-    expect(context.fromUri(Uri.parse('file:///')), '/');
-    expect(context.fromUri(Uri.parse('foo/bar')), 'foo/bar');
-    expect(context.fromUri(Uri.parse('/path/to/foo')), '/path/to/foo');
-    expect(context.fromUri(Uri.parse('///path/to/foo')), '/path/to/foo');
-    expect(context.fromUri(Uri.parse('file:///path/to/foo%23bar')),
-        '/path/to/foo#bar');
-    expect(context.fromUri(Uri.parse('_%7B_%7D_%60_%5E_%20_%22_%25_')),
-        r'_{_}_`_^_ _"_%_');
-    expect(() => context.fromUri(Uri.parse('http://dartlang.org')),
-        throwsArgumentError);
+  group('fromUri', () {
+    test('with a URI', () {
+      expect(context.fromUri(Uri.parse('file:///path/to/foo')), '/path/to/foo');
+      expect(context.fromUri(Uri.parse('file:///path/to/foo/')),
+          '/path/to/foo/');
+      expect(context.fromUri(Uri.parse('file:///')), '/');
+      expect(context.fromUri(Uri.parse('foo/bar')), 'foo/bar');
+      expect(context.fromUri(Uri.parse('/path/to/foo')), '/path/to/foo');
+      expect(context.fromUri(Uri.parse('///path/to/foo')), '/path/to/foo');
+      expect(context.fromUri(Uri.parse('file:///path/to/foo%23bar')),
+          '/path/to/foo#bar');
+      expect(context.fromUri(Uri.parse('_%7B_%7D_%60_%5E_%20_%22_%25_')),
+          r'_{_}_`_^_ _"_%_');
+      expect(() => context.fromUri(Uri.parse('http://dartlang.org')),
+          throwsArgumentError);
+    });
+
+    test('with a string', () {
+      expect(context.fromUri('file:///path/to/foo'), '/path/to/foo');
+    });
   });
 
   test('toUri', () {