`idlharness.js` incorrectly tests that `Promise<>`-returning methods throw.

[mikewest]

Given an IDL like:

interface Whatever {
    Promise<boolean> method(...);
};

idlharness checks that Whatever::method() throws TypeError when it's called with incorrect parameters. It should instead check that the returned Promise is rejected with a TypeError.

[jgraham]

#161 perhaps?

[zhaoming0]

@jgraham when we test http://w3c-test.org/webrtc/rtcpeerconnection/rtcpeerconnection-idl.html RTCPeerConnection interface: operation setLocalDescription(RTCSessionDescription,VoidFunction,RTCPeerConnectionErrorCallback)

The Promise attribute has throw:
assert_throws: calling operation with this = null didn't throw TypeError function "function () { fn.apply(obj, args); }" did not throw

Promise parsed "operation.idlType.generic == null"

Could you help to check it thanks

[wpt-issue-mover]

This issue has been moved to web-platform-tests/wpt#7164; please continue all discussion there.