Assert that DOM structs have the correct first field

DOM structs embed their parent type as their first field. This
introduces a `.parent()` method to the DOM struct that returns its first
field, and codegens a type assert that ensures that `.parent()` returns
the parent struct.

This generates:

On `#[dom_struct]`:

```rust
impl HasParent for Type {
    type Parent = ParentType;
    fn as_parent(&self) -> ParentType {
        &self.first_field
    }
}
```

In the codegen files:

```rust
impl Type {
    fn __assert_parent_type(&self) {
        let _: &ParentType = self.as_parent();
    }
}
````

1 files changed, 3 insertions, 4 deletions

diff --git a/components/script/dom/filereadersync.rs b/components/script/dom/filereadersync.rs
index cc9a821a073..10095db4961 100644
--- a/components/script/dom/filereadersync.rs
+++ b/components/script/dom/filereadersync.rs
@@ -5,11 +5,10 @@
 use dom::bindings::codegen::Bindings::BlobBinding::BlobMethods;
 use dom::bindings::codegen::Bindings::FileReaderSyncBinding::{FileReaderSyncBinding, FileReaderSyncMethods};
 use dom::bindings::error::{Error, Fallible};
-use dom::bindings::reflector::reflect_dom_object;
+use dom::bindings::reflector::{Reflector, reflect_dom_object};
 use dom::bindings::root::DomRoot;
 use dom::bindings::str::DOMString;
 use dom::blob::Blob;
-use dom::eventtarget::EventTarget;
 use dom::filereader::FileReaderSharedFunctionality;
 use dom::globalscope::GlobalScope;
 use dom_struct::dom_struct;
@@ -20,13 +19,13 @@ use std::ptr::NonNull;
 
 #[dom_struct]
 pub struct FileReaderSync {
-    eventtarget: EventTarget,
+    reflector: Reflector,
 }
 
 impl FileReaderSync {
     pub fn new_inherited() -> FileReaderSync {
         FileReaderSync {
-            eventtarget: EventTarget::new_inherited(),
+            reflector: Reflector::new(),
         }
     }