Coding style. Add the reason for using 'var' and not 'final'.

Change-Id: Id30cad3b8c5e45d290d6f1cf5bdc79320e6ff0b7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/362900
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

pkg/analyzer/doc/implementation/coding_style.md

diff --git a/pkg/analyzer/doc/implementation/coding_style.md b/pkg/analyzer/doc/implementation/coding_style.md
index 0fb919ed..ca39afa 100644
--- a/pkg/analyzer/doc/implementation/coding_style.md
+++ b/pkg/analyzer/doc/implementation/coding_style.md
@@ -91,6 +91,10 @@
 - We use `final` if the local variable shadows a field and is being used to
   allow the type of the field to be promoted.
 
+The reason to prefer `var` over `final` is that the majority of the team felt
+that the benefit of using `var` was slightly greater than the benefit of
+using `final`.
+
 ## Styles we're discussing
 
 This section documents the styles we are currently discussing. The purpose