There seems to be an issue with parsing composite character sequences.

Composite character İ consist of the base character and combining character sequence (U+0049)(U+0307)
Using this character causing the compiler to fail with the parser error.

Example code

var bar = {
İ: "foo"
};

Compiler error

input0:2: ERROR - [JSC_PARSE_ERROR] Parse error. Character '̇' (U+0307) is not a valid identifier start char
  2| İ: "foo"
      ^

1 error(s), 0 warning(s)

Demo

Example