Update the coding_style doc

After lengthy consideration, I believe that this is the best policy for
the team overall. It also corresponds to what I thought we had already
agreed to in the past.

Change-Id: I952297bf966ace0bdb105a5e4539420c3c6b8983
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/345748
Commit-Queue: Brian Wilkerson <[email protected]>
Reviewed-by: Phil Quitslund <[email protected]>
Reviewed-by: Samuel Rawlins <[email protected]>

pkg/analyzer/doc/implementation/coding_style.md

@@ -67,6 +67,19 @@ named `FooExtension`. While this can lead to long names (like
 `AnalysisOptionsImplExtension`), the extension name is rarely used (only in
 explicit extension overrides), and we value consistency.
 
+### Modifier usage
+
+#### Local variables
+
+We use `var` to declare local variables with two exceptions:
+
+- We use a type annotation if the type of the variable would be incorrectly
+  inferred without it, such as when a variable that needs to be nullable would
+  be inferred to be non-nullable because it's initialized to a non-null value.
+
+- We use `final` if the local variable shadows a field and is being used to
+  allow the type of the field to be promoted.
+
 ## Styles we're discussing
 
 This section documents the styles we are currently discussing. The purpose