Error: fullwidth low line (U+FF3F) is not valid in identifier

The fullwidth low line (U+FF3F) character should be valid to use in a Javascript identifier but the compiler throws a parse error. [Example](https://closure-compiler-debugger.appspot.com/#input0%3Dclass%2520MyClass%2520%257B%2520constructor()%2520%257B%2520this.foo%25EF%25BC%25BF1%253B%2520%257D%2520%257D%250A%26input1%26conformanceConfig%26externs%26refasterjs-template%26includeDefaultExterns%3Dtrue%26CHECK_SYMBOLS%3Dtrue%26MISSING_PROPERTIES%3Dtrue%26TRANSPILE%3Dtrue%26CHECK_TYPES%3Dtrue%26CLOSURE_PASS%3Dtrue%26PRESERVE_TYPE_ANNOTATIONS%3Dtrue%26PRETTY_PRINT%3Dtrue).

The spec states that:

> any Unicode code point with the Unicode property “ID_Continue”, “Other_ID_Continue”, or “Other_ID_Start”

should be valid.