Fix decorator this memoization

[JLHwung]

Q	A
Fixed Issues?	When a class decorator is an member expression with non-static parent object, Babel fails to memoise the parent object when generating this value for the decoration helper. (REPL)
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

In this PR we fix a regression introduced in #16218. In the following example

"use strict";

function dec() {}

let o1 = { dec };
let o4oCounter = 0;
let o4 = { o() { o4oCounter++; return o1 } };

@(o4.o().dec)
class A {}

console.log(o4oCounter);

The o4oCounter should be 1 because o4.o is only called once. However it currently logs 2 because Babel does not memoise o4.o() when generating class decorators:

_classDecs = [o4.o(), o4.o().dec];

[nicolo-ribaudo]

Here if ,array was a local variable we wouldn't memoize it, right?

[nicolo-ribaudo]

Right

[JLHwung]

Yes, we will memoize it when it is a const reference

babel/packages/babel-traverse/src/scope/index.ts

Lines 557 to 572 in bfefe70

	isStatic(node: t.Node): boolean {
	if (isThisExpression(node) || isSuper(node) || isTopicReference(node)) {
	return true;
	}
	if (isIdentifier(node)) {
	const binding = this.getBinding(node.name);
	if (binding) {
	return binding.constant;
	} else {
	return this.hasBinding(node.name);
	}
	}
	return false;
	}

I think the pureGetters could extend to global variables as well, so when this assumption is true, we don't memoise global variables as well as member expressions.