macro implementation doesn't guarantee detection of templates before attempting to apply them

[jcollins-g]

Even the line here: https://github.com/dart-lang/dartdoc/blob/master/lib/dartdoc.dart#L180 only changes the order in which things are loaded. The unfortunately named _rawDocs are cached by ModelElement's documentation so after this initial pass we'll never see the templates again.

This has always been a problem, but the canonicalization overhaul made this worse because before #1368 we usually recreated ModelElements frequently enough to cloak this (which caused its own problems). Fix with multiple passes or changing how templates work to make them no longer inline (but sourced from a separate file).

Lower priority only because macros don't seem to be in heavy use yet.

[jcollins-g]

Moving to P2 because AFAICT there isn't even a warning when this happens; can demote to P3 if we at least warn on macros not being found.

[srawlins]

Closing this as stale. At least flutter has a good system for warning on un-replaced macros.