Issue with br labels in the JS build

Somehow, a module in text format like

(module
  (type $t0 (func (param i32)))
  (type $t1 (func))
  (import "env" "log" (func $log (type $t0)))
  (func $main (type $t1)
    block $B0
      block $B1
        br $B1
        i32.const 3
        call $log
      end
      i32.const 2
      call $log
    end
    i32.const 1
    call $log)
  (start 1))

becomes

(module
  (type $t0 (func (param i32)))
  (type $t1 (func))
  (import "env" "log" (func $log (type $t0)))
  (func $main (type $t1)
    block $B0
      block $B1
        br 3228196 (; INVALID ;)
        i32.const 3
        call $log
      end
      i32.const 2
      call $log
    end
    i32.const 1
    call $log)
  (start 1))

after parsing with libwabt.js. Note the invalid pointer value instead of the label on the br. Any ideas?

See: https://github.com/reklatsmasters/vscode-wasm/issues/14#issuecomment-423853330

Start by reproducing the WAT example through libwabt.js and compare the parsed or printed output around br $B1. Trace the text-parser and JavaScript build entry points involved in branch-label handling; done means the label is preserved in the generated text instead of becoming an invalid pointer value, with a regression test covering this input.