UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:64.0) Gecko/20100101 Firefox/64.0

Steps to reproduce the problem:
1.Open Developer tool.
2.Paste the below code.

function zg(x) {
  a: if (!x) b: { alert("foo"); }
  else if (x && x.$classData) b: {
    if (null !== x) {
      alert("foo"); 
      break b;
    }
  }
}

3. Run `zg("test")`

What is the expected behavior?
Chrome should not raise SyntaxError and execute the code, since the code seems valid on the ECMAScript spec.
http://www.ecma-international.org/ecma-262/#sec-labelled-statements-static-semantics-containsduplicatelabels

Other browsers (Firefox and Edge) can execute without Syntax Error.

What went wrong?
Chrome raises the following error.

Uncaught SyntaxError: Label 'b' has already been declared
    at <anonymous>:1:1

Did this work before? N/A 

Chrome version: 71.0.3578.98 (Official Build) (64 bit) (cohort: Stable)  Channel: stable
OS Version: 10.0
Flash Version: 32.0.0.101

The code is minimum example.
Original code was generated by Scala.js and optimized by Google Closure Compiler on advanced optimization mode.

Please refer original issue reported on Scala.js repository.
https://github.com/scala-js/scala-js/issues/3508