Parsing a helper expression that uses the ~ operator throws

[matthewp]

Test case in b91adc4

var exprData = expression.parse('each ~foo');

This throws with:

TypeError: Cannot read property '0' of undefined
    at Object.hydrateAst (http://localhost/can-stache/src/expression.js:659:47)
    at Object.hydrateAst (http://localhost/can-stache/src/expression.js:688:23)
    at Object.parse (http://localhost/can-stache/src/expression.js:647:19)
    at Object.<anonymous> (http://localhost/can-stache/test/expression-test.js:690:28)