Remove sorting of allowedHelp maps (#852)

Closes #845

It is idiomatic to treat the key order of a Dart map as meaningful
given that map literals and default Map type preserve key insertion
order. It is more useful to allow the caller to decide this order than
to mandate an alpha sorting by key. Callers which need this order can
construct the map appropriately, and callers which prefer a different
order now have the capability.

Remove the additional list copy and iterate the map keys directly.

Releasing as a non-breaking change since specific usage output is
considered an implementation detail. This is expected to impact some CI
statuf for packages with tests hardcoding a strict dependency on the
output.

No additional tests are necessary since updating the order in existing
tests demonstrates the same behavior as adding a non-sorting specific
test.

Refactor a few null check conditions to variable assignment patterns.

pkgs/args/CHANGELOG.md

@@ -1,5 +1,7 @@
 ## 2.6.1-wip
 
+* Remove sorting of the `allowedHelp` argument in usage output. Ordering will
+  depend on key order for the passed `Map`.
 * Fix the repository URL in `pubspec.yaml`.
 * Added option `hideNegatedUsage` to `ArgParser.flag()` allowing a flag to be
   `negatable` without showing it in the usage text.

pkgs/args/lib/src/arg_parser.dart

@@ -177,6 +177,12 @@ class ArgParser {
   ///
   /// The [allowedHelp] argument is a map from values in [allowed] to
   /// documentation for those values that will be included in [usage].
+  /// The map may include a subset of the allowed values.
+  /// Additional values that are not in [allowed] should be omitted, however
+  /// there is no validation.
+  /// When both [allowed] and [allowedHelp] are passed, only [allowed] will
+  /// be validated at parse time, and only [allowedHelp] will be included in
+  /// usage output.
   ///
   /// The [defaultsTo] argument indicates the value this option will have if the
   /// user doesn't explicitly pass it in (or `null` by default).
@@ -231,6 +237,12 @@ class ArgParser {
   ///
   /// The [allowedHelp] argument is a map from values in [allowed] to
   /// documentation for those values that will be included in [usage].
+  /// The map may include a subset of the allowed values.
+  /// Additional values that are not in [allowed] should be omitted, however
+  /// there is no validation.
+  /// When both [allowed] and [allowedHelp] are passed, only [allowed] will
+  /// be validated at parse time, and only [allowedHelp] will be included in
+  /// usage output.
   ///
   /// The [defaultsTo] argument indicates the values this option will have if
   /// the user doesn't explicitly pass it in (or `[]` by default).

pkgs/args/lib/src/usage.dart

@@ -87,15 +87,13 @@ class _Usage {
     _write(0, _abbreviation(option));
     _write(1, '${_longOption(option)}${_mandatoryOption(option)}');
 
-    if (option.help != null) _write(2, option.help!);
+    if (option.help case final help?) _write(2, help);
 
-    if (option.allowedHelp != null) {
-      var allowedNames = option.allowedHelp!.keys.toList();
-      allowedNames.sort();
+    if (option.allowedHelp case final allowedHelp?) {
       _newline();
-      for (var name in allowedNames) {
+      for (var MapEntry(key: name, value: content) in allowedHelp.entries) {
         _write(1, _allowedTitle(option, name));
-        _write(2, option.allowedHelp![name]!);
+        _write(2, content);
       }
       _newline();
     } else if (option.allowed != null) {

pkgs/args/test/usage_test.dart

@@ -216,10 +216,10 @@ void main() {
       validateUsage(parser, '''
           --suit              Like in cards
 
+                [spades]      Swords of a soldier
                 [clubs]       Weapons of war
                 [diamonds]    Money for this art
                 [hearts]      The shape of my heart
-                [spades]      Swords of a soldier
           ''');
     });
 
@@ -244,10 +244,10 @@ void main() {
       validateUsage(parser, '''
           --suit                     Like in cards
 
+                [spades]             Swords of a soldier
                 [clubs] (default)    Weapons of war
                 [diamonds]           Money for this art
                 [hearts]             The shape of my heart
-                [spades]             Swords of a soldier
           ''');
     });
 
@@ -271,10 +271,10 @@ void main() {
       validateUsage(parser, '''
           --suit                      Like in cards
 
+                [spades]              Swords of a soldier
                 [clubs] (default)     Weapons of war
                 [diamonds]            Money for this art
                 [hearts] (default)    The shape of my heart
-                [spades]              Swords of a soldier
           ''');
     });