Commits on Jul 3, 2018

1. 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();
       }
   }
   ````

   

   Manishearth committed Jul 3, 2018

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