[diagnostics] use_build_context_synchronously message fix

Picking up abandoned change: https://dart-review.googlesource.com/c/sdk/+/395461

Change-Id: Id5a876dec73870bb379e9ad89226669272ebf673
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/396382
Commit-Queue: Phil Quitslund <pquitslund@google.com>
Reviewed-by: Marya Belanger <mbelanger@google.com>

pkg/analyzer/tool/diagnostics/diagnostics.md

@@ -29271,7 +29271,7 @@ The analyzer produces this diagnostic when a `BuildContext` is referenced
 by a `StatefulWidget` after an asynchronous gap without first checking the
 `mounted` property.
 
-Storing a `BuildContext` for later use can lead to difficult to diagnose
+Storing a `BuildContext` for later use can lead to difficult-to-diagnose
 crashes. Asynchronous gaps implicitly store a `BuildContext`, making them
 easy to overlook for diagnosis.
 

pkg/linter/messages.yaml

@@ -13161,7 +13161,7 @@ LintCode:
       by a `StatefulWidget` after an asynchronous gap without first checking the
       `mounted` property.
 
-      Storing a `BuildContext` for later use can lead to difficult to diagnose
+      Storing a `BuildContext` for later use can lead to difficult-to-diagnose
       crashes. Asynchronous gaps implicitly store a `BuildContext`, making them
       easy to overlook for diagnosis.
 
@@ -13213,7 +13213,7 @@ LintCode:
     deprecatedDetails: |-
       **DON'T** use `BuildContext` across asynchronous gaps.
 
-      Storing `BuildContext` for later usage can easily lead to difficult to diagnose
+      Storing `BuildContext` for later usage can easily lead to difficult-to-diagnose
       crashes. Asynchronous gaps are implicitly storing `BuildContext` and are some of
       the easiest to overlook when writing code.