fix Picture.toImage return type check and api conform test.

lib/stub_ui/lib/src/ui/canvas.dart

@@ -978,7 +978,7 @@ class Picture {
   ///
   /// Although the image is returned synchronously, the picture is actually
   /// rasterized the first time the image is drawn and then cached.
-  Image toImage(int width, int height) => null;
+  Future<Image> toImage(int width, int height) => null;
 
   /// Release the resources used by this object. The object is no longer usable
   /// after this method is called.

web_sdk/test/api_conform_test.dart

@@ -148,6 +148,15 @@ void main() {
               '${uiParam.identifier.name} is named, but not in lib/stub_ui/ui.dart.');
         }
       }
+      // check return type.
+      if (uiMethod.returnType?.toString() != webMethod.returnType?.toString()) {
+        // allow dynamic in web implementation.
+        if (webMethod.returnType?.toString() != 'dynamic') {
+          failed = true;
+          print('Warning: lib/ui/ui.dart $className.$methodName return type'
+              '${uiMethod.returnType?.toString()} is not the same as in lib/stub_ui/ui.dart.');
+        }
+      }
     }
   }
   if (failed) {
@@ -158,8 +167,6 @@ void main() {
   exit(0);
 }
 
-void _checkParameters() {}
-
 // Collects all public classes defined by the part files of [unit].
 void _collectPublicClasses(CompilationUnit unit,
     Map<String, ClassDeclaration> destination, String root) {