fix(mock-doc): add hasAttribute to MockCSSStyleDeclaration

[simonhaenisch]

Proposed changes:

• add hasAttribute implementation to MockCSSStyleDeclaration

---

Without this I sometimes run into the following error with jest@26:

PrettyFormatPluginError: _val$hasAttribute.call is not a functionTypeError: _val$hasAttribute.call is not a function

      at testNode (node_modules/jest-matcher-utils/node_modules/pretty-format/build/plugins/DOMElement.js:32:27)

The reason this happens is because the css style declaration is proxied, and the ProxyHandler's get method returns an empty string for unknown properties:

stencil/src/mock-doc/css-style-declaration.ts

Line 74 in a1c320b

return cssStyle.getPropertyValue(prop);

stencil/src/mock-doc/css-style-declaration.ts

Lines 14 to 17 in a1c320b

	getPropertyValue(prop: string) {
	prop = jsCaseToCssCase(prop);
	return String(this._styles.get(prop) || '');
	}

When jest tries to pretty-format, it checks whether the object has a hasAttribute property, and if it does, it assumes that it's a function and tries calling it (which fails because the proxy returns a string '').

---

My first assumption was that MockCSSStyleDeclaration is supposed to be the mock for the <style> element, in which case I thought it might also make sense that it extends MockHTMLElement so that all element-related methods are implemented. But now I see that this is regarding the style attribute, so maybe it makes sense to get this fixed in pretty-format instead:

https://github.com/facebook/jest/blob/7430a7824421c122cd07035d800d22e1007408fa/packages/pretty-format/src/plugins/DOMElement.ts#L30

- val.hasAttribute?.('is');
+ typeof val.hasAttribute === 'function' && val.hasAttribute('is');

(pretty-format is a bit optimistic here to assume that hasAttribute is always callable like this)

[simonhaenisch]

Just a quick update, my PR for fixing this in Jest's pretty-format has been merged, so this should also soon be solved by using a newer jest version. I'll close this PR for now.