[null-safety] fix type declaration of Picutre._toImage (#21942)

Jonah Williams · web-flow · commit 1c35cc6416ed · 2020-10-17T11:02:19.000-07:00
Fixes flutter/flutter#68377 Should be a nullable string. Looks like --null-assertions does not cover the native binding code.
diff --git a/lib/ui/painting.dart b/lib/ui/painting.dart
@@ -4689,7 +4689,7 @@ class Picture extends NativeFieldWrapperClass2 {
     );
   }
 
-  String _toImage(int width, int height, _Callback<_Image> callback) native 'Picture_toImage';
+  String? _toImage(int width, int height, _Callback<_Image> callback) native 'Picture_toImage';
 
   /// Release the resources used by this object. The object is no longer usable
   /// after this method is called.

Original file line number	Diff line number	Diff line change
`@@ -4689,7 +4689,7 @@ class Picture extends NativeFieldWrapperClass2 {`
`4689`	`4689`	`);`
`4690`	`4690`	`}`
`4691`	`4691`
`4692`		`- String _toImage(int width, int height, _Callback<_Image> callback) native 'Picture_toImage';`
	`4692`	`+ String? _toImage(int width, int height, _Callback<_Image> callback) native 'Picture_toImage';`
`4693`	`4693`
`4694`	`4694`	`/// Release the resources used by this object. The object is no longer usable`
`4695`	`4695`	`/// after this method is called.`