FlowTypeCheck internal failure

[DavePearce]

Test 001135 is this:

type neg is (int n) where n < 0
type pos is (int n) where n > 0

type exp1 is neg | {exp1 rest}
type exp2 is pos | {exp2 rest}

function f(exp1 e1) -> exp2:
    return (exp2) e1

This currently generates an internal failure:

wycc.lang.Syntactic$Exception
	at wyil.check.FlowTypeCheck.internalFailure(FlowTypeCheck.java:3486)
	at wyil.check.FlowTypeCheck.checkStatement(FlowTypeCheck.java:406)
	at wyil.check.FlowTypeCheck.checkBlock(FlowTypeCheck.java:330)
	at wyil.check.FlowTypeCheck.checkFunctionOrMethodDeclaration(FlowTypeCheck.java:272)
	at wyil.check.FlowTypeCheck.checkDeclaration(FlowTypeCheck.java:210)
	at wyil.check.FlowTypeCheck.checkUnit(FlowTypeCheck.java:223)
	at wyil.check.FlowTypeCheck.checkDeclaration(FlowTypeCheck.java:194)
	at wyil.check.FlowTypeCheck.check(FlowTypeCheck.java:182)