Promise.resolve should consider side-effect in thenables

[lukastaegert]

• Rollup Version: 1.14.5
• Operating System (or Browser): any
• Node Version: any

How Do We Reproduce?

Taken from es6-shim:

var count = 0;
var thenable = Object.defineProperty({}, 'then', { get: function () { count += 1; } });
Promise.resolve(thenable);
assert.equal(count, 1);

Expected Behavior

Promise.resolve is retained if a thenable is provided that has a side-effect when then is accessed.

Actual Behavior

Promise.resolve is removed.
https://rollupjs.org/repl?version=1.14.6&shareable=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

[lukastaegert]

Closing in favour of #4102