Fix analyzer warnings in http_multi_server.

pkg/http_multi_server/lib/src/multi_headers.dart

Index: pkg/http_multi_server/lib/src/multi_headers.dart
diff --git a/pkg/http_multi_server/lib/src/multi_headers.dart b/pkg/http_multi_server/lib/src/multi_headers.dart
index 1677a9fa8cb806da979226b2773820f238f43f29..fcd782cfce699e3eb76bc0d4b7a78b2aa945ef98 100644
--- a/pkg/http_multi_server/lib/src/multi_headers.dart
+++ b/pkg/http_multi_server/lib/src/multi_headers.dart
@@ -95,13 +95,13 @@ class MultiHeaders implements HttpHeaders {
 
   void remove(String name, Object value) {
     for (var headers in _headers) {
-      headers.remove(name);
+      headers.remove(name, value);
     }
   }
 
   void removeAll(String name) {
     for (var headers in _headers) {
-      headers.removeAll(name, value);
+      headers.removeAll(name);
     }
   }
 
@@ -114,4 +114,10 @@ class MultiHeaders implements HttpHeaders {
   String value(String name) => _headers.first.value(name);
 
   List<String> operator[](String name) => _headers.first[name];
+
+  void clear() {
+    for (var headers in _headers) {
+      headers.clear();
+    }
+  }
 }