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

pkg/path/lib/src/context.dart

Index: pkg/path/lib/src/context.dart
diff --git a/pkg/path/lib/src/context.dart b/pkg/path/lib/src/context.dart
index b897c8ba53310d6a890c258bc5821c08b14427c5..9c8e1e46465b489ccfa7c13d6610d80c0562b489 100644
--- a/pkg/path/lib/src/context.dart
+++ b/pkg/path/lib/src/context.dart
@@ -423,23 +423,30 @@ class Context {
     return parsed.toString();
   }
 
-  /// Returns the path represented by [uri].
+  /// Returns the path represented by [uri], which may be a [String] or a [Uri].
   ///
   /// For POSIX and Windows styles, [uri] must be a `file:` URI. For the URL
   /// style, this will just convert [uri] to a string.
   ///
   ///     // POSIX
-  ///     context.fromUri(Uri.parse('file:///path/to/foo'))
+  ///     context.fromUri('file:///path/to/foo')
   ///       // -> '/path/to/foo'
   ///
   ///     // Windows
-  ///     context.fromUri(Uri.parse('file:///C:/path/to/foo'))
+  ///     context.fromUri('file:///C:/path/to/foo')
   ///       // -> r'C:\path\to\foo'
   ///
   ///     // URL
-  ///     context.fromUri(Uri.parse('http://dartlang.org/path/to/foo'))
+  ///     context.fromUri('http://dartlang.org/path/to/foo')
   ///       // -> 'http://dartlang.org/path/to/foo'
-  String fromUri(Uri uri) => style.pathFromUri(uri);
+  ///
+  /// If [uri] is relative, a relative path will be returned.
+  ///
+  ///     path.fromUri('path/to/foo'); // -> 'path/to/foo'
+  String fromUri(uri) {
+    if (uri is String) uri = Uri.parse(uri);
+    return style.pathFromUri(uri);
+  }
 
   /// Returns the URI that represents [path].
   ///