add preserveHeaderCase for classes extends HttpHeaders (#49766)

packages/flutter_test/lib/src/_binding_io.dart

@@ -499,7 +499,7 @@ class _MockHttpHeaders extends HttpHeaders {
   List<String> operator [](String name) => <String>[];
 
   @override
-  void add(String name, Object value) { }
+  void add(String name, Object value, {bool preserveHeaderCase = false}) { }
 
   @override
   void clear() { }
@@ -517,7 +517,7 @@ class _MockHttpHeaders extends HttpHeaders {
   void removeAll(String name) { }
 
   @override
-  void set(String name, Object value) { }
+  void set(String name, Object value, {bool preserveHeaderCase = false}) { }
 
   @override
   String value(String name) => null;

packages/flutter_tools/test/src/testbed.dart

@@ -622,7 +622,7 @@ class FakeHttpHeaders extends HttpHeaders {
   List<String> operator [](String name) => <String>[];
 
   @override
-  void add(String name, Object value) { }
+  void add(String name, Object value, {bool preserveHeaderCase = false}) { }
 
   @override
   void clear() { }
@@ -640,7 +640,7 @@ class FakeHttpHeaders extends HttpHeaders {
   void removeAll(String name) { }
 
   @override
-  void set(String name, Object value) { }
+  void set(String name, Object value, {bool preserveHeaderCase = false}) { }
 
   @override
   String value(String name) => null;