In JS, function declarations should allow subsequent prototype assignment

[sandersn]

// @allowJs: true
// @checkJs: true
// @Filename: a.js
function C(p) {
  this.p = p
}
C.prototype = {
  m() {
    console.log(this.p)
  }
}
C.prototype.q = function(r) {
  return this.p === r
}
var c = new C(1)
c.q(2)
c.m()

Expected behavior:
Both q and m are methods on c.

Actual behavior:
Only q is a method on c; m is not recognised.

Note that if the declaration is var C = function(p) { this.p = p }, everything works. This only happens when C is a function declaration instead of a variable declaration with a function initialiser.

[sandersn]

Another repro from axios:

function E() {
  this.message = 'String contains an invalid character';
}
E.prototype = new Error;
E.prototype.code = 5;
E.prototype.name = 'InvalidCharacterError';