Polymer pass crashes when "properties" is not an object literal

https://closure-compiler-debugger.appspot.com/#input0%3DPolymer(%257B%250A%2520%2520is%253A%2520'x-y'%252C%250A%2520%2520properties%253A%2520(function()%2520%257B%250A%2520%2520%2520%2520return%2520%257B%257D%253B%250A%2520%2520%257D)()%252C%250A%257D)%26input1%26conformanceConfig%26externs%3Dvar%2520Polymer%2520%253D%2520function(obj)%2520%257B%257D%250Avar%2520PolymerElement%2520%253D%2520function(obj)%2520%257B%257D%26refasterjs-template%26includeDefaultExterns%3D1%26CHECK_SYMBOLS%3D1%26CHECK_TYPES%3D1%26CLOSURE_PASS%3D1%26LANG_IN_IS_ES6%3D1%26MISSING_PROPERTIES%3D1%26POLYMER_PASS%3D1%26PRESERVE_TYPE_ANNOTATIONS%3D1%26PRETTY_PRINT%3D1%26TRANSPILE%3D1

We don't want to allow this, I don't think. But we should report an error rather than crashing :)