Generating binding for __wbindgen_object_drop_ref isn't optimized out

[alexcrichton]

Currently on wasm-bindgen a bare file like:

#[wasm_bindgen]
pub fn foo() {}

will generate the JS:

import * as wasm from './add_bg';                               
                                                                
/**                                                             
* @returns {void}                                               
*/                                                              
export function foo() {                                         
    return wasm.foo();                                          
}                                                               
                                                                
const heap = new Array(32);                                     
                                                                
heap.fill(undefined);                                           
                                                                
heap.push(undefined, null, true, false);                        
                                                                
let heap_next = heap.length;                                    
                                                                
function dropObject(idx) {                                      
    if (idx < 36) return;                                       
    heap[idx] = heap_next;                                      
    heap_next = idx;                                            
}                                                               
                                                                
export function __wbindgen_object_drop_ref(i) { dropObject(i); }

but __wbindgen_object_drop_ref isn't referenced by the wasm file! Something in our passes likely needs to be reordered (aka generate bindings for __wbindgen_object_drop_ref after the gc pass), but we should understand what's happening before we do this as well.