Develop selector validators compatible with spec

[mgechev]

Currently the selectors' validators are not compatible with spec.

[mgechev]

During the last meeting, we decided to use the CssSelector and it's parse method from @angular/compiler. This way we'll be able to have semantical check based on the selector type.

For instance, for @Components the check should look like:

const selector = CssSelector.parse(component.selector);
if (selector.element !== null) {
  // validate if it is a valid kebab-case elector, if not, throw a warning
} else {
  // throw a warning that the component doesn’t use element selector
}

Problem that needs to be considered if a mixture is used (i.e. component with element and attribute/class selector, etc.). We need to make sure we're aligned with the style guide.

[preslavsh]

#137 Needs reviewing

[mgechev]

Fixed by 3373dff.