The [[Delete]] internal method doesn't need a receiver, so the deleteProperty trap only has two arguments. See 9.5.10-6 https://tc39.github.io/ecma262/#sec-proxy-object-internal-methods-and-internal-slots-delete-p

Able to reproduce the issue on Mac 10.13.3, Win-10 and Ubuntu 14.04 using chrome reported version #66.0.3359.181 and latest canary #68.0.3436.0.
This is a non-regression issue as it is observed from M60 old builds. 

Hence, marking it as untriaged to get more inputs from dev team.

Thanks...!!

Then it's a specification bug because it's impossible to use proxies with methods.

I've updated the example to show that within a class method:

  method() {
    // this here is the receiver, not the target
    // hence there's no way to use proxies with classes
    wm.get(this).push(`invoked method`);
  }

FYI I have also raised the question in the ECMAScript repository:
https://github.com/tc39/ecma262/issues/1198

AFAIK this is a wont fix with a better workaround than a dual weak map, as explained in here: https://github.com/tc39/ecma262/issues/1198#issuecomment-390804560

```
const handlersProto = {
  deleteProperty(target, key) {
    console.log({ proxy: this.proxy });
    return Reflect.deleteProperty(target, key);
  }
};

class Foo {
  constructor() {
    const handlers = Object.create(handlersProto);
    const proxy = (handlers.proxy = new Proxy(this, handlers));
    wm.set(proxy, []);
    return proxy;
  }
}
```