migrate update_icons to null safety (#84063)

dev/tools/update_icons.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 // Regenerates the material icons file.
 // See https://github.com/flutter/flutter/wiki/Updating-Material-Design-Fonts-&-Icons
 
@@ -332,7 +330,7 @@ class _Icon {
     id = tokenPair.key;
     hexCodepoint = tokenPair.value;
 
-    if (id.endsWith('_outlined') && id!='insert_chart_outlined') {
+    if (id.endsWith('_outlined') && id != 'insert_chart_outlined') {
       shortId = _replaceLast(id, '_outlined');
       htmlSuffix = '-outlined';
     } else if (id.endsWith('_rounded')) {
@@ -349,7 +347,7 @@ class _Icon {
     flutterId = id;
     for (final MapEntry<String, String> rewritePair in identifierRewrites.entries) {
       if (id.startsWith(rewritePair.key)) {
-        flutterId = id.replaceFirst(rewritePair.key, identifierRewrites[rewritePair.key]);
+        flutterId = id.replaceFirst(rewritePair.key, identifierRewrites[rewritePair.key]!);
       }
     }
 
@@ -358,14 +356,14 @@ class _Icon {
 
   static const List<String> styleSuffixes = <String>['', '_outlined', '_rounded', '_sharp'];
 
-  String id;            // e.g. 5g, 5g_outlined, 5g_rounded, 5g_sharp
-  String shortId;       // e.g. 5g
-  String flutterId;     // e.g. five_g, five_g_outlined, five_g_rounded, five_g_sharp
-  String name;          // e.g. five g, five g outlined, five g rounded, five g sharp
-  String hexCodepoint;  // e.g. e547
+  late String id;            // e.g. 5g, 5g_outlined, 5g_rounded, 5g_sharp
+  late String shortId;       // e.g. 5g
+  late String flutterId;     // e.g. five_g, five_g_outlined, five_g_rounded, five_g_sharp
+  late String name;          // e.g. five g, five g outlined, five g rounded, five g sharp
+  late String hexCodepoint;  // e.g. e547
 
   // The suffix for the 'material-icons' HTML class.
-  String htmlSuffix;
+  late String htmlSuffix;
 
   String get mirroredInRTL => _iconsMirroredWhenRTL.contains(shortId) ? ', matchTextDirection: true' : '';
 
@@ -394,9 +392,8 @@ class _Icon {
     // Sort a regular icon before its variants.
     if (shortId == b.shortId) {
       return id.length - b.id.length;
-    } else {
-      return flutterId.compareTo(b.flutterId);
     }
+    return flutterId.compareTo(b.flutterId);
   }
 
   String _replaceLast(String string, String toReplace) {