Silently outputting invalid assembly code for a nonsense program

[FlatAssembler]

For this nonsense AEC code:

Function foo(Integer32 a, Integer32 b) Which Returns Integer32 Does
  Return invertBits(a := b);
EndFunction

The compiler outputs the following assembly code:

;;Generated by AECforWebAssembly ( https://github.com/FlatAssembler/AECforWebAssembly ).
(module 
  (import "JavaScript" "memory" (memory 1)) ;;Assume that, in the JavaScript program linked to this
  ;;AEC program, there is an "extern"-ed bytearray of size 1 page (64KB) called "memory".
  ;;Have a better idea?
  (global $stack_pointer (import "JavaScript" "stack_pointer") (mut i32)) ;;The stack pointer being
  ;;visible from JavaScript will be useful when debugging the compiler. This breaks compatibility with
  ;;WebAssembly environments which do not support "WebAssembly.Global", such as Firefox 52 (the last
  ;;version of Firefox to run on Windows XP, and the first one supporting WebAssembly).
  (func $foo (param i32) (param i32) (result i32) (local $return_value i32)
    (global.set $stack_pointer ;;Allocate the space for the arguments of that function on the system stack.
      (i32.add
        (global.get $stack_pointer)
        (i32.const 8)
      )
    )
    (i32.store 
      (i32.sub
        (global.get $stack_pointer)
        (i32.const 8) ;;a
      )
      (local.get 0)
    )
    (i32.store 
      (i32.sub
        (global.get $stack_pointer)
        (i32.const 4) ;;b
      )
      (local.get 1)
    )
    ;;Setting for returning: (invertBits (:= a b))
    (local.set $return_value
      (i32.xor (i32.const -1)
        ;;Assigning b to a.
        (i32.store
          (i32.sub
            (global.get $stack_pointer)
            (i32.const 8) ;;a
          )
          (i32.load
            (i32.sub
              (global.get $stack_pointer)
              (i32.const 4) ;;b
            )
          )
        )
      )
    )
    (global.set $stack_pointer (i32.sub (global.get $stack_pointer) (i32.const 8))) ;;Cleaning up the system stack before returning.
    (return (local.get $return_value))
    (global.set $stack_pointer (i32.sub (global.get $stack_pointer) (i32.const 8)))
    (i32.const 0)
  )
  (export "foo" (func $foo))
)

That is invalid assembly code, the assembler wat2wasm from WebAssembly Binary Toolkit has this to say about it:

debug.wat:33:8: error: type mismatch in i32.xor, expected [i32, i32] but got [i32]
      (i32.xor (i32.const -1)
       ^^^^^^^

It has to do with the way I implemented named arguments. Namely, when the semantic analyzer sees the := being used inside a built-in (therefore, in its mind, already declared) function, it assumes it is a named argument of that function and passes the nonsensical code to the compiler. Compiler then assumes it is a normal assignment directive (not checking that it is passed as an argument to a built-in function that is invertBits), and outputs invalid assembly code because of that. There does not seem to be a simple solution. We can perhaps modify the semantic analyzer to disallow := being used as an argument to built-in functions, but, then, whenever we add another built-in function, we would need to modify that (or else it will be outdated).