Add a shortcut for PATCH.

lib/src/client.dart

Index: lib/src/client.dart
diff --git a/lib/src/client.dart b/lib/src/client.dart
index e29a9e0c31a6df8a142b2bae80f7c84fc970ba3a..eaeccfef6bdccb8aa8f618ad7fdf9a5e4f458528 100644
--- a/lib/src/client.dart
+++ b/lib/src/client.dart
@@ -18,7 +18,7 @@ import 'streamed_response.dart';
 /// The interface for HTTP clients that take care of maintaining persistent
 /// connections across multiple requests to the same server. If you only need to
 /// send a single request, it's usually easier to use [head], [get], [post],
-/// [put], or [delete] instead.
+/// [put], [patch], or [delete] instead.
 ///
 /// When creating an HTTP client class with additional functionality, you must
 /// extend [BaseClient] rather than [Client]. In most cases, you can wrap
@@ -89,6 +89,27 @@ abstract class Client {
   Future<Response> put(url, {Map<String, String> headers, body,
       Encoding encoding});
 
+  /// Sends an HTTP PATCH request with the given headers and body to the given
+  /// URL, which can be a [Uri] or a [String].
+  ///
+  /// [body] sets the body of the request. It can be a [String], a [List<int>]
+  /// or a [Map<String, String>]. If it's a String, it's encoded using
+  /// [encoding] and used as the body of the request. The content-type of the
+  /// request will default to "text/plain".
+  ///
+  /// If [body] is a List, it's used as a list of bytes for the body of the
+  /// request.
+  ///
+  /// If [body] is a Map, it's encoded as form fields using [encoding]. The
+  /// content-type of the request will be set to
+  /// `"application/x-www-form-urlencoded"`; this cannot be overridden.
+  ///
+  /// [encoding] defaults to [UTF8].
+  ///
+  /// For more fine-grained control over the request, use [send] instead.
+  Future<Response> patch(url, {Map<String, String> headers, body,
+      Encoding encoding});
+
   /// Sends an HTTP DELETE request with the given headers to the given URL,
   /// which can be a [Uri] or a [String].
   ///