Internal Compiler Error with ES6 class extends in externs but not code

Not sure what is the culprit here, but the following externs (while not great) break the compiler (and make it ask me to report this error, so here) -
```
class RequestEntityInit extends RequestInit {
/** @param {?} value */
set bodyEntity(value) {}
/** @return {?} */
get bodyEntity() {}
};
```

https://closure-compiler.appspot.com/home#code%3D%252F%252F%2520%253D%253DClosureCompiler%253D%253D%250A%252F%252F%2520%2540compilation_level%2520ADVANCED_OPTIMIZATIONS%250A%252F%252F%2520%2540output_file_name%2520default.js%250A%252F%252F%2520%2540js_externs%2520class%2520RequestEntityInit%2520extends%2520RequestInit%2520%257B%2520%252F**%2520%2540param%2520%257B%253F%257D%2520value%2520*%252F%2520set%2520bodyEntity(value)%2520%257B%257D%2520%2520%252F**%2520%2540return%2520%257B%253F%257D%2520*%252F%2520get%2520bodyEntity()%2520%257B%257D%2520%257D%250A%252F%252F%2520%2540language_out%2520ES5%250A%252F%252F%2520%253D%253D%252FClosureCompiler%253D%253D%250A

```
java.lang.RuntimeException: INTERNAL COMPILER ERROR.
Please report this problem.

Unexpected variable $jscomp
Node(NAME $jscomp): web/externs.js:7:0
class RequestEntityInit extends RequestInit {
Parent(GETPROP): web/externs.js:7:0
class RequestEntityInit extends RequestInit {

at com.google.javascript.jscomp.VarCheck.visit(VarCheck.java:222)
at com.google.javascript.jscomp.NodeTraversal.traverseBranch(NodeTraversal.java:806)
at com.google.javascript.jscomp.NodeTraversal.traverseChildren(NodeTraversal.java:877)
at com.google.javascript.jscomp.NodeTraversal.traverseBranch(NodeTraversal.java:802)
at com.google.javascript.jscomp.NodeTraversal.traverseChildren(NodeTraversal.java:877)
at com.google.javascript.jscomp.NodeTraversal.traverseBranch(NodeTraversal.java:802)
at com.google.javascript.jscomp.NodeTraversal.traverseChildren(NodeTraversal.java:877)
at com.google.javascript.jscomp.NodeTraversal.traverseBranch(NodeTraversal.java:802)
at com.google.javascript.jscomp.NodeTraversal.traverseChildren(NodeTraversal.java:877)
at com.google.javascript.jscomp.NodeTraversal.traverseBranch(NodeTraversal.java:802)
at com.google.javascript.jscomp.NodeTraversal.traverseChildren(NodeTraversal.java:877)
at com.google.javascript.jscomp.NodeTraversal.traverseBranch(NodeTraversal.java:802)
at com.google.javascript.jscomp.NodeTraversal.traverseChildren(NodeTraversal.java:877)
at com.google.javascript.jscomp.NodeTraversal.handleScript(NodeTraversal.java:758)
at com.google.javascript.jscomp.NodeTraversal.traverseBranch(NodeTraversal.java:783)
at com.google.javascript.jscomp.NodeTraversal.traverseChildren(NodeTraversal.java:877)
at com.google.javascript.jscomp.NodeTraversal.traverseBranch(NodeTraversal.java:802)
at com.google.javascript.jscomp.NodeTraversal.traverseRoots(NodeTraversal.java:356)
at com.google.javascript.jscomp.VarCheck.process(VarCheck.java:145)
at com.google.javascript.jscomp.PhaseOptimizer$NamedPass.process(PhaseOptimizer.java:303)
at com.google.javascript.jscomp.PhaseOptimizer.process(PhaseOptimizer.java:230)
at com.google.javascript.jscomp.Compiler.performOptimizations(Compiler.java:2480)
at com.google.javascript.jscomp.Compiler$3.call(Compiler.java:808)
at com.google.javascript.jscomp.Compiler$3.call(Compiler.java:804)
at com.google.javascript.jscomp.CompilerExecutor$2.call(CompilerExecutor.java:102)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.IllegalStateException: Unexpected variable $jscomp
... 29 more
```