Revert accidentally added "skip".

Author: davidmorgan

source_gen/test/type_checker_test.dart

@@ -182,7 +182,6 @@ void main() {
         onPlatform: const {
           'windows': Skip('https://github.com/dart-lang/source_gen/issues/573'),
         },
-        skip: 'Google3 test gives google3:// URI for the test mirror',
       );
     });
 
@@ -200,14 +199,10 @@ void main() {
     group(
       '(MapMixin',
       () {
-        test(
-          'should equal MapMixin class',
-          () {
-            expect(checkMapMixin().isExactlyType(staticMapMixin), isTrue);
-            expect(checkMapMixin().isExactly2(staticMapMixin.element3), isTrue);
-          },
-          skip: 'Google3 test gives google3:// URI for the test mirror',
-        );
+        test('should equal MapMixin class', () {
+          expect(checkMapMixin().isExactlyType(staticMapMixin), isTrue);
+          expect(checkMapMixin().isExactly2(staticMapMixin.element3), isTrue);
+        });
       },
       onPlatform: const {
         'windows': Skip('https://github.com/dart-lang/source_gen/issues/573'),