Add flutter-note site shortcut (#4134)

Makes progress towards #3720

_config.yml

@@ -105,6 +105,9 @@ alert:
   note: >-
     <aside class="alert alert-info" role="alert" markdown="1">
     <i class="fas fa-info-circle"></i> **Note:**
+  flutter-note: >-
+    <aside class="alert alert-info" role="alert" markdown="1">
+    <img src="/assets/img/shared/flutter/icon/64.png" width="24" alt="Flutter logo"> **Flutter note:**
   version-note: >-
     <aside class="alert alert-info" role="alert" markdown="1">
     <i class="fas fa-code-branch"></i> **Version note:**

src/_guides/language/concurrency/index.md

@@ -302,8 +302,7 @@ This section discusses some examples
 that use the `Isolate` API
 to implement isolates.
 
-{{site.alert.info}}
-  **Flutter note:**
+{{site.alert.flutter-note}}
   If you're using Flutter on a non-web platform,
   then instead of using the `Isolate` API directly,
   consider using the [Flutter `compute()` function][].
@@ -459,10 +458,9 @@ is slower when isolates are in different groups.
 
 [`Isolate.spawnUri()`]: {{site.dart-api}}/{{site.data.pkg-vers.SDK.channel}}/dart-isolate/Isolate/spawnUri.html
 
-{{ site.alert.info }}
-  **Flutter note:**
+{{site.alert.flutter-note}}
   Flutter doesn't support `Isolate.spawnUri()`.
-{{ site.alert.end }}
+{{site.alert.end}}
 
 {% comment %}
 TODO:

src/_tutorials/libraries/shared-pkgs.md

@@ -34,8 +34,7 @@ just change the package name when you get to that step.
 This tutorial also describes some of the resources you can expect to find
 in a well-built package.
 
-{{site.alert.info}}
-  **Flutter note:**
+{{site.alert.flutter-note}}
   This page doesn't describe the tools you use with Flutter, but the
   concepts are the same, and you can share packages between
   your Flutter and web or server-side apps.

src/tools/pub/pubspec.md

@@ -83,8 +83,8 @@ A pubspec can have the following fields:
 
 Pub ignores all other fields,
 
-{{site.alert.info}}
-  **Flutter note:** Pubspecs for [Flutter apps]({{site.flutter}}) can have
+{{site.alert.flutter-note}}
+  Pubspecs for [Flutter apps]({{site.flutter}}) can have
   [additional fields]({{site.flutter-docs}}/development/tools/pubspec)
   for configuring the environment and managing assets.
 {{site.alert.end}}
@@ -307,8 +307,7 @@ platforms:
   macos:
 ```
 
-{{site.alert.info}}
-  **Flutter note:**
+{{site.alert.flutter-note}}
   Flutter plugins use [plugin declarations][]
   instead of this field.
 {{site.alert.end}}