Only overwrite content when resolving import.meta properties

[lukastaegert]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

Description

A small issue I stumbled across when working on #2938. As import meta properties are resolved AFTER everything else is rendered it is possible due to some quirks of magic-string that content is removed that should not be removed. For instance this code:

export const url = import.meta.url;

would become in SystemJS

System.register('bundle', [], function (exports, module) {
	'use strict';
	return {
		execute: function () {

			const url = exports('url', 'url'

		}
	};
});

Notice the missing closing bracket. This issue fixes this by making import.meta properties a "contentOnly" update, which should prevent other issues as well.