Keep track of references to guide.

pkg/front_end/lib/src/fasta/diagnostics.md

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 # Guide for Writing Diagnostics
 
 ## The Rule of 3
 
 A great message conveys the following three things:
 
 1. What is wrong?
 2. Why is it wrong?
 3. How do I fix it?
 
@@ skipping 59 matching lines @@
 
 Versus:
 
     "Change the return type." // Avoid this.
 
 Notice that the style of the language in which this guide is written, is mostly imperative. That's not an example to follow when writing diagnostics.
 
 ## Other Resources
 
 One language and community where good error messages have been discussed intensively is [Elm](http://elm-lang.org/blog/compiler-errors-for-humans).