Enhance Finder toString() (#3458)

* Enhance Finder toString()

packages/flutter_test/lib/src/widget_tester.dart

@@ -291,7 +291,7 @@ abstract class Finder {
   }
 
   @override
-  String toString() => 'widget with $description';
+  String toString() => '[Finder for $description]';
 }
 
 /// Indicates that an attempt to find a widget within the current element tree

packages/flutter_test/test/widget_tester_test.dart

@@ -26,9 +26,9 @@ void main() {
 
         expect(failure, isNotNull);
         String message = failure.message;
-        expect(message, contains('Expected: widget with text "foo" exists in the element tree'));
+        expect(message, contains('Expected: [Finder for text "foo"] exists in the element tree'));
         expect(message, contains("Actual: <Instance of 'WidgetTester'>"));
-        expect(message, contains('Which: Does not contain widget with text "foo"'));
+        expect(message, contains('Which: Does not contain [Finder for text "foo"]'));
       });
     });
   });
@@ -53,9 +53,9 @@ void main() {
 
         expect(failure, isNotNull);
         String message = failure.message;
-        expect(message, contains('Expected: widget with text "foo" does not exist in the element tree'));
+        expect(message, contains('Expected: [Finder for text "foo"] does not exist in the element tree'));
         expect(message, contains("Actual: <Instance of 'WidgetTester'>"));
-        expect(message, contains('Which: Contains widget with text "foo"'));
+        expect(message, contains('Which: Contains [Finder for text "foo"]'));
       });
     });
   });