dart: Fix incorrect write in Float64 write method (#8290)

Co-authored-by: Llamadmiral <Llamadmiral@users.noreply.github.com>
Co-authored-by: Derek Bailey <derekbailey@google.com>

dart/test/flat_buffers_test.dart

@@ -215,6 +215,10 @@ class BuilderTest {
       ..addTestarrayofstringOffset(testArrayOfString);
     final mon = monBuilder.finish();
     fbBuilder.finish(mon);
+
+    final mon3 = example.Monster(fbBuilder.buffer);
+    expect(mon3.name, 'MyMonster');
+    expect(mon3.pos!.test1, 3.0);
   }
 
   void test_error_addInt32_withoutStartTable([Builder? builder]) {