elements schema: detect dangling attributes

In the ideal world we should be able to detect “dangling” attributes. In the <div foo> example we should report foo as “dangling” when:

• foo is not a valid HTML attribute (how come should we identify valid attributes? From a spec / schema?)
• there is no matching directive
• it wasn’t consumed by a binding / event handler / var declaration / other ng-specific construct
• it is not prefixed with data- (?)

In practice “dangling” attributes in templates are most probably attributes are introduced due to:

• typos
• forgetting to add a directive (in the directives for the @View annotation) that should match a given attribute

[jelbourn]

Per #25005, this should now track, at least, that doing something like

host: {
  'attr.aria-label': 'Hello'
}

should throw an error (missing [ ])

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