Object literal methods in prototype assignments don't have correct this-type

[sandersn]

// @noImplicitAny: true
/**
 * @constructor
 * @template {string} K
 * @template V
 */
// function Multimap() {
var Multimap = function() {
    /** @type {Object<string, V>} TODO: Remove the prototype from the fresh object */
    this._map = {};
};

Multimap.prototype = {
  /**
   * @param {K} key
   * @param {V} value
   */
  set: function(key, value) {
    var set = this._map.get(key);
    if (!set) {
      set = new Set();
      this._map.set(key, set);
    }
    set.add(value);
  }
}

Expected behavior:

No errors on this._map references.

Actual behavior:

Property '_map' doesn't exist on type { set(key: K): V }.

[sandersn]

This is a regression in 3.1. Doesn't repro on 3.0.

[sandersn]

The reason 3.0 doesn't repro is that all objects have an index signature, even with noImplicitAny. In 3.1, noImplicitAny doesn't put an index signature on object literals.

Outside references to Multimap instances have the correct type because they create an intersection. checkThisExpression should do the same thing.