JS Promise partial support

[karray]

It seems that js promises only works partially.

Can't create a promise from Python:

from js import window

window.Promise(lambda resolve: resolve())
# TypeError: undefined is not a promise

promise.finally not working:

// JS
window.fetch('https://github.com/iodide-project/pyodide').then(resp => console.log(resp.status)).finally(()=>console.log('Done'))

#python
from js import window

window.fetch('https://github.com/iodide-project/pyodide').then(lambda resp: print(resp.status)).finally(lambda: print('done'))

SyntaxError: invalid syntax

    at _hiwire_throw_error (pyodide.asm.js:8)
    at _pythonexc2js (pyodide-cdn2.iodide.io/v0.15.0/full/pyodide.asm.wasm:wasm-function[358]:0xfc9e1)
    at __findImports (pyodide-cdn2.iodide.io/v0.15.0/full/pyodide.asm.wasm:wasm-function[386]:0xfe795)
    at Object.Module.__findImports (pyodide.asm.js:8)
    at Object.Module.runPythonAsync (pyodide.asm.js:8)
    at evaluatePython (index.html:41)
    at HTMLButtonElement.onclick (index.html:17)

[rth]

For Promise the error is also more explicit in Firefox,

TypeError: calling a builtin Promise constructor without new is forbidden

Here, unlike #768 importing from js works fine,

>>> from js import Promise
>>> Promise.new(lambda resolve, err: resolve())

About finally, unfortunately it's a reserved key word in Python, so it cannot be a method name. So for instance,

class A:
    def finally(self):
        return 0

produces,

    def finally(self):
        ^
SyntaxError: invalid syntax

in Python (even outside Pyodide). Let's see if anyone has ideas how to address this on the CPython side (https://stackoverflow.com/questions/64396988).

[hoodmane]

PR #880 wraps the promise in a Future so we could expose that API, but it's a bit different, and a bit less usable, and doesn't have an equivalent to finally.
We should expose the finally method on the proxied future at a slightly different name. eval_code should be updated to look for syntax errors triggered by finally and tell people about the moved API.

[hoodmane]

@dalcde Do you have any opinion about what name we should use for the promise finally method on JsProxy?

[hoodmane]

Closed by #1334.