Accessing non-existent field in constructor causes failed assertion

[plietar]

actor Main
    new create(env: Env) =>
        this.x = None

Building builtin -> /Users/paul/Projects/ponyc/packages/builtin
Building . -> /private/tmp/a
src/libponyc/pass/refer.c:636: is_lvalue: Assertion `def != NULL` failed.

Backtrace:
  0   ponyc                               0x000000010ed61a41 ponyint_assert_fail + 161
  1   ponyc                               0x000000010ed5b190 is_lvalue + 560
  2   ponyc                               0x000000010ed59470 refer_assign + 176
  3   ponyc                               0x000000010ed58f15 pass_refer + 165
  4   ponyc                               0x000000010ed1b20d ast_visit + 685
  5   ponyc                               0x000000010ed1b144 ast_visit + 484
  6   ponyc                               0x000000010ed1b144 ast_visit + 484
  7   ponyc                               0x000000010ed1b144 ast_visit + 484
  8   ponyc                               0x000000010ed1b144 ast_visit + 484
  9   ponyc                               0x000000010ed1b144 ast_visit + 484
  10  ponyc                               0x000000010ed1b144 ast_visit + 484
  11  ponyc                               0x000000010ed1b144 ast_visit + 484
  12  ponyc                               0x000000010ed1bb0c visit_pass + 156
  13  ponyc                               0x000000010ed1b5bf ast_passes + 671
  14  ponyc                               0x000000010ed1b315 ast_passes_program + 37
  15  ponyc                               0x000000010ed388ae program_load + 190
  16  ponyc                               0x000000010ec7dc1f compile_package + 47
  17  ponyc                               0x000000010ec7da50 main + 1120
  18  libdyld.dylib                       0x00007fff8f773255 start + 1
  19  ???                                 0x0000000000000001 0x0 + 1

cc @jemc, this seems related to ##1715 and #1867.