JSON export fails, despite lack of visible errors, manual compilation succeeds

Encountered while verifying https://github.com/inkle/ink/issues/344 against Unity.

The code from that issue, reproduced below, works in the preview pane, and has no errors or warnings, but JSON export fails with the following message:

`Could not export: Ink has errors - please fix them before exporting.`

Manually invoking inklecate produced a JSON file with behavior identical to the preview pane.

Code:
```
~ temp val = 1
~ temp val2 = 5
initial values: {val}, {val2}
-> deref(-> load_1, val, val2) ->
final values: {val}, {val2}

VAR MY_VALUE_1 = 10

=== load_1(ref val, msg) ===

pre set {msg}: {val}
~ val = MY_VALUE_1
post set {msg}: {val}
->->

=== deref(-> func1, ref out, ref out2) ===

~ temp my_value_1 = 2
-> func1(out, "parameter") -> func1(my_value_1, "temp") ->
post-divert parameter: {out}
post-divert temp: {my_value_1}
constant divert:
-> load_1(out2, "constant") ->
->->
```