Closure Compiler's type warnings show an incorrect type error when using a symbol on the left side of an in expression:

src/js/SourceFile:100: WARNING - left side of 'in'
found   : symbol
required: string
        if ( SYMBOL in record ) {
              ^^^^^^^^^

This should be updated to allow either a string or symbol on the left side of in.

(Using --compilation_level=ADVANCED_OPTIMIZATIONS, --language_out=ECMASCRIPT5_STRICT, and Closure Compiler version 20180805