Simplify arguments to outdated (#2402)

diff --git a/lib/src/command/outdated.dart b/lib/src/command/outdated.dart
index fcfc6b6..1b94bf8 100644
--- a/lib/src/command/outdated.dart
+++ b/lib/src/command/outdated.dart
@@ -32,11 +32,13 @@
   String get docUrl => 'https://dart.dev/tools/pub/cmd/pub-outdated';
 
   OutdatedCommand() {
-    argParser.addOption('format',
-        help: 'Defines how the output should be formatted. Defaults to color '
-            'when connected to a terminal, and no-color otherwise.',
-        valueHelp: 'FORMAT',
-        allowed: ['color', 'no-color', 'json']);
+    argParser.addFlag('color',
+        help: 'Whether to color the output. Defaults to color '
+            'when connected to a terminal, and no-color otherwise.');
+
+    argParser.addFlag('json',
+        help: 'Outputs the results in a json formatted report',
+        negatable: false);
 
     argParser.addFlag('up-to-date',
         defaultsTo: false,
@@ -152,11 +154,11 @@
 
     rows.sort();
 
-    if (argResults['format'] == 'json') {
+    if (argResults['json']) {
       await _outputJson(rows);
     } else {
-      final useColors = argResults['format'] == 'color' ||
-          (!argResults.wasParsed('format') && stdin.hasTerminal);
+      final useColors = argResults['color'] ||
+          (!argResults.wasParsed('color') && stdin.hasTerminal);
       final marker = {
         'outdated': oudatedMarker,
         'none': noneMarker,
diff --git a/test/outdated/goldens/circular_dependencies.txt b/test/outdated/goldens/circular_dependencies.txt
index 7f53199..2b04839 100644
--- a/test/outdated/goldens/circular_dependencies.txt
+++ b/test/outdated/goldens/circular_dependencies.txt
@@ -1,4 +1,4 @@
-$ pub outdated --format=json
+$ pub outdated --json
 Resolving...
 {
   "packages": [
@@ -12,7 +12,7 @@
   ]
 }
 
-$ pub outdated --format=no-color
+$ pub outdated --no-color
 Resolving...
 Package  Current  Upgradable  Resolvable  Latest  
 dependencies
@@ -24,7 +24,7 @@
 1 upgradable dependency is locked (in pubspec.lock) to an older version.
 To update it, use `pub upgrade`.
 
-$ pub outdated --format=no-color --mark=none
+$ pub outdated --no-color --mark=none
 Resolving...
 Package  Current  Upgradable  Resolvable  Latest  
 dependencies
@@ -36,7 +36,7 @@
 1 upgradable dependency is locked (in pubspec.lock) to an older version.
 To update it, use `pub upgrade`.
 
-$ pub outdated --format=no-color --up-to-date
+$ pub outdated --no-color --up-to-date
 Resolving...
 Package  Current  Upgradable  Resolvable  Latest  
 dependencies
@@ -48,7 +48,7 @@
 1 upgradable dependency is locked (in pubspec.lock) to an older version.
 To update it, use `pub upgrade`.
 
-$ pub outdated --format=no-color --pre-releases
+$ pub outdated --no-color --pre-releases
 Resolving...
 Package  Current  Upgradable  Resolvable  Latest  
 dependencies
@@ -60,7 +60,7 @@
 1 upgradable dependency is locked (in pubspec.lock) to an older version.
 To update it, use `pub upgrade`.
 
-$ pub outdated --format=no-color --no-dev-dependencies
+$ pub outdated --no-color --no-dev-dependencies
 Resolving...
 Package  Current  Upgradable  Resolvable  Latest  
 dependencies
diff --git a/test/outdated/goldens/mutually_incompatible.txt b/test/outdated/goldens/mutually_incompatible.txt
index b88a3e1..7942438 100644
--- a/test/outdated/goldens/mutually_incompatible.txt
+++ b/test/outdated/goldens/mutually_incompatible.txt
@@ -1,4 +1,4 @@
-$ pub outdated --format=json
+$ pub outdated --json
 Resolving...
 {
   "packages": [
@@ -19,7 +19,7 @@
   ]
 }
 
-$ pub outdated --format=no-color
+$ pub outdated --no-color
 Resolving...
 Package  Current  Upgradable  Resolvable  Latest  
 dependencies
@@ -33,7 +33,7 @@
 Dependencies are all on the latest resolvable versions.
 Newer versions, while available, are not mutually compatible.
 
-$ pub outdated --format=no-color --mark=none
+$ pub outdated --no-color --mark=none
 Resolving...
 Package  Current  Upgradable  Resolvable  Latest  
 dependencies
@@ -47,7 +47,7 @@
 Dependencies are all on the latest resolvable versions.
 Newer versions, while available, are not mutually compatible.
 
-$ pub outdated --format=no-color --up-to-date
+$ pub outdated --no-color --up-to-date
 Resolving...
 Package  Current  Upgradable  Resolvable  Latest  
 dependencies
@@ -61,7 +61,7 @@
 Dependencies are all on the latest resolvable versions.
 Newer versions, while available, are not mutually compatible.
 
-$ pub outdated --format=no-color --pre-releases
+$ pub outdated --no-color --pre-releases
 Resolving...
 Package  Current  Upgradable  Resolvable  Latest  
 dependencies
@@ -75,7 +75,7 @@
 Dependencies are all on the latest resolvable versions.
 Newer versions, while available, are not mutually compatible.
 
-$ pub outdated --format=no-color --no-dev-dependencies
+$ pub outdated --no-color --no-dev-dependencies
 Resolving...
 Package  Current  Upgradable  Resolvable  Latest  
 dependencies
diff --git a/test/outdated/goldens/newer_versions.txt b/test/outdated/goldens/newer_versions.txt
index 7322203..8051f8d 100644
--- a/test/outdated/goldens/newer_versions.txt
+++ b/test/outdated/goldens/newer_versions.txt
@@ -1,4 +1,4 @@
-$ pub outdated --format=json
+$ pub outdated --json
 Resolving...
 {
   "packages": [
@@ -40,7 +40,7 @@
   ]
 }
 
-$ pub outdated --format=no-color
+$ pub outdated --no-color
 Resolving...
 Package      Current  Upgradable  Resolvable  Latest  
 dependencies 
@@ -60,7 +60,7 @@
 4  dependencies are constrained to versions that are older than a resolvable version.
 To update these dependencies, edit pubspec.yaml.
 
-$ pub outdated --format=no-color --mark=none
+$ pub outdated --no-color --mark=none
 Resolving...
 Package      Current  Upgradable  Resolvable  Latest  
 dependencies 
@@ -80,7 +80,7 @@
 4  dependencies are constrained to versions that are older than a resolvable version.
 To update these dependencies, edit pubspec.yaml.
 
-$ pub outdated --format=no-color --up-to-date
+$ pub outdated --no-color --up-to-date
 Resolving...
 Package        Current  Upgradable  Resolvable  Latest  
 dependencies   
@@ -102,7 +102,7 @@
 4  dependencies are constrained to versions that are older than a resolvable version.
 To update these dependencies, edit pubspec.yaml.
 
-$ pub outdated --format=no-color --pre-releases
+$ pub outdated --no-color --pre-releases
 Resolving...
 Package      Current  Upgradable  Resolvable  Latest       
 dependencies 
@@ -122,7 +122,7 @@
 4  dependencies are constrained to versions that are older than a resolvable version.
 To update these dependencies, edit pubspec.yaml.
 
-$ pub outdated --format=no-color --no-dev-dependencies
+$ pub outdated --no-color --no-dev-dependencies
 Resolving...
 Package     Current  Upgradable  Resolvable  Latest  
 dependencies
diff --git a/test/outdated/goldens/no_dependencies.txt b/test/outdated/goldens/no_dependencies.txt
index 3d52184..dd288c8 100644
--- a/test/outdated/goldens/no_dependencies.txt
+++ b/test/outdated/goldens/no_dependencies.txt
@@ -1,26 +1,26 @@
-$ pub outdated --format=json
+$ pub outdated --json
 Resolving...
 {
   "packages": []
 }
 
-$ pub outdated --format=no-color
+$ pub outdated --no-color
 Resolving...
 Found no outdated packages.
 
-$ pub outdated --format=no-color --mark=none
+$ pub outdated --no-color --mark=none
 Resolving...
 Found no outdated packages.
 
-$ pub outdated --format=no-color --up-to-date
+$ pub outdated --no-color --up-to-date
 Resolving...
 Found no outdated packages.
 
-$ pub outdated --format=no-color --pre-releases
+$ pub outdated --no-color --pre-releases
 Resolving...
 Found no outdated packages.
 
-$ pub outdated --format=no-color --no-dev-dependencies
+$ pub outdated --no-color --no-dev-dependencies
 Resolving...
 Found no outdated packages.
 
diff --git a/test/outdated/outdated_test.dart b/test/outdated/outdated_test.dart
index ef4ba37..63b658e 100644
--- a/test/outdated/outdated_test.dart
+++ b/test/outdated/outdated_test.dart
@@ -13,12 +13,12 @@
 Future<void> variations(String name) async {
   final buffer = StringBuffer();
   for (final args in [
-    ['--format=json'],
-    ['--format=no-color'],
-    ['--format=no-color', '--mark=none'],
-    ['--format=no-color', '--up-to-date'],
-    ['--format=no-color', '--pre-releases'],
-    ['--format=no-color', '--no-dev-dependencies'],
+    ['--json'],
+    ['--no-color'],
+    ['--no-color', '--mark=none'],
+    ['--no-color', '--up-to-date'],
+    ['--no-color', '--pre-releases'],
+    ['--no-color', '--no-dev-dependencies'],
   ]) {
     final process = await startPub(args: ['outdated', ...args]);
     await process.shouldExit(0);