refactor(DirectiveMetadata): make _hostRegExp private - #2694

@vicb this LGTM but I wonder if this is needed. This var is not exported so it is file-private (at least in JS!) - which is what we need, right? I guess special care needs to be taken for Dart, right? Since in JS no-one could import it anyway (?).

Dart would export any non private ("_" prefixed) var

see #2695

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