Relax the warning of unavailable tokens in gen_defaults when a default value is provided (#140872)

Fixes some false positives and false negatives from the `gen_defaults` tool regarding unavailable tokens in the Material spec.

Fixes #140871 

# Before

![image](https://github.com/flutter/flutter/assets/22084723/eb6c43ee-d919-4203-80ee-e36869e5bbcf)

# After

![image](https://github.com/flutter/flutter/assets/22084723/67093dcb-1ab0-439f-9338-a6f364d2a9e1)

These new issues that are logged are being fixed in https://github.com/flutter/flutter/pull/140573

---

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

dev/tools/gen_defaults/lib/input_decorator_template.dart

@@ -215,10 +215,10 @@ class _${blockName}DefaultsM3 extends InputDecorationTheme {
         ? componentColor(componentToken1)
         : componentColor(componentToken2);
     final double width = (
-        getToken('$componentToken1.width') ??
-        getToken('$componentToken1.height') ??
-        getToken('$componentToken2.width') ??
-        getToken('$componentToken2.height') ??
+        getToken('$componentToken1.width', optional: true) ??
+        getToken('$componentToken1.height', optional: true) ??
+        getToken('$componentToken2.width', optional: true) ??
+        getToken('$componentToken2.height', optional: true) ??
         1.0) as double;
     return 'BorderSide(color: $borderColor${width != 1.0 ? ", width: $width" : ""})';
   }

dev/tools/gen_defaults/lib/template.dart

@@ -38,7 +38,11 @@ abstract class TokenTemplate {
   bool tokenAvailable(String tokenName) => _tokens.containsKey(tokenName);
 
   /// Resolve a token while logging its usage.
-  dynamic getToken(String tokenName) {
+  /// There will be no log if [optional] is true and the token doesn't exist.
+  dynamic getToken(String tokenName, {bool optional = false}) {
+    if (optional && !tokenAvailable(tokenName)) {
+      return null;
+    }
     tokenLogger.log(tokenName);
     return _tokens[tokenName];
   }
@@ -108,14 +112,17 @@ abstract class TokenTemplate {
   /// If there is a value for the given token, this will return
   /// the value prepended with [colorSchemePrefix].
   ///
-  /// Otherwise it will return [defaultValue].
+  /// Otherwise it will return [defaultValue] if provided or 'null' if not.
+  ///
+  /// If a [defaultValue] is not provided and the token doesn't exist, the token
+  /// lookup is logged and a warning will be shown at the end of the process.
   ///
   /// See also:
   ///   * [componentColor], that provides support for an optional opacity.
-  String color(String colorToken, [String defaultValue = 'null']) {
-    return tokenAvailable(colorToken)
-      ? '$colorSchemePrefix${getToken(colorToken)}'
-      : defaultValue;
+  String color(String colorToken, [String? defaultValue]) {
+    final String effectiveDefault = defaultValue ?? 'null';
+    final dynamic tokenVal = getToken(colorToken, optional: defaultValue != null);
+    return tokenVal == null ? effectiveDefault : '$colorSchemePrefix$tokenVal';
   }
 
   /// Generate a [ColorScheme] color name for the given token or a transparent
@@ -244,7 +251,11 @@ abstract class TokenTemplate {
       return 'null';
     }
     final String borderColor = componentColor(componentToken);
-    final double width = (getToken('$componentToken.width') ?? getToken('$componentToken.height') ?? 1.0) as double;
+    final double width = (
+      getToken('$componentToken.width', optional: true) ??
+      getToken('$componentToken.height', optional: true) ??
+      1.0
+    ) as double;
     return 'BorderSide(color: $borderColor${width != 1.0 ? ", width: $width" : ""})';
   }
 

dev/tools/gen_defaults/lib/token_logger.dart

@@ -28,16 +28,20 @@ class TokenLogger {
   // Sorted set of used tokens.
   final SplayTreeSet<String> _usedTokens = SplayTreeSet<String>();
 
+  // Set of tokens that were referenced on some templates, but do not exist.
+  final Set<String> _unavailableTokens = <String>{};
+
   void clear() {
     _allTokens.clear();
     _versionMap.clear();
     _usedTokens.clear();
+    _unavailableTokens.clear();
   }
 
   /// Logs a token.
   void log(String token) {
     if (!_allTokens.containsKey(token)) {
-      print('\x1B[31m' 'Token unavailable: $token' '\x1B[0m');
+      _unavailableTokens.add(token);
       return;
     }
     _usedTokens.add(token);
@@ -76,6 +80,14 @@ class TokenLogger {
     }
 
     print('Tokens used: ${_usedTokens.length}/${_allTokens.length}');
+
+    if (_unavailableTokens.isNotEmpty) {
+      print('');
+      print('\x1B[31m' 'Some referenced tokens do not exist: ${_unavailableTokens.length}' '\x1B[0m');
+      for (final String token in _unavailableTokens) {
+        print('  $token');
+      }
+    }
   }
 
   /// Dumps version and tokens usage to a file.

dev/tools/gen_defaults/test/gen_defaults_test.dart

@@ -325,10 +325,69 @@ static final String tokenBar = 'foo';
       logger.log('foobar');
       logger.printTokensUsage(verbose: true);
 
-      expect(printLog, contains(errorColoredString('Token unavailable: baz')));
-      expect(printLog, contains(errorColoredString('Token unavailable: foobar')));
       expect(printLog, contains('❌ foo'));
       expect(printLog, contains('Tokens used: 0/1'));
+      expect(printLog, contains(errorColoredString('Some referenced tokens do not exist: 2')));
+      expect(printLog, contains('  baz'));
+      expect(printLog, contains('  foobar'));
+    }));
+
+    test("color function doesn't log when providing a default", overridePrint(() {
+      allTokens['color_foo_req'] = 'value';
+
+      // color_foo_opt is not available, but because it has a default value, it won't warn about it
+
+      TestColorTemplate('block', 'filename', allTokens).generate();
+      logger.printTokensUsage(verbose: true);
+
+      expect(printLog, contains('✅ color_foo_req'));
+      expect(printLog, contains('Tokens used: 1/1'));
+    }));
+
+    test('color function logs when not providing a default', overridePrint(() {
+      // Nor color_foo_req or color_foo_opt are available, but only color_foo_req will be logged.
+      // This mimics a token being removed, but expected to exist.
+
+      TestColorTemplate('block', 'filename', allTokens).generate();
+      logger.printTokensUsage(verbose: true);
+
+      expect(printLog, contains('Tokens used: 0/0'));
+      expect(printLog, contains(errorColoredString('Some referenced tokens do not exist: 1')));
+      expect(printLog, contains('  color_foo_req'));
+    }));
+
+    test('border function logs width token when available', overridePrint(() {
+      allTokens['border_foo.color'] = 'red';
+      allTokens['border_foo.width'] = 3.0;
+
+      TestBorderTemplate('block', 'filename', allTokens).generate();
+      logger.printTokensUsage(verbose: true);
+
+      expect(printLog, contains('✅ border_foo.color'));
+      expect(printLog, contains('✅ border_foo.width'));
+      expect(printLog, contains('Tokens used: 2/2'));
+    }));
+
+    test('border function logs height token when width token not available', overridePrint(() {
+      allTokens['border_foo.color'] = 'red';
+      allTokens['border_foo.height'] = 3.0;
+
+      TestBorderTemplate('block', 'filename', allTokens).generate();
+      logger.printTokensUsage(verbose: true);
+
+      expect(printLog, contains('✅ border_foo.color'));
+      expect(printLog, contains('✅ border_foo.height'));
+      expect(printLog, contains('Tokens used: 2/2'));
+    }));
+
+    test("border function doesn't log when width or height tokens not available", overridePrint(() {
+      allTokens['border_foo.color'] = 'red';
+
+      TestBorderTemplate('block', 'filename', allTokens).generate();
+      logger.printTokensUsage(verbose: true);
+
+      expect(printLog, contains('✅ border_foo.color'));
+      expect(printLog, contains('Tokens used: 1/1'));
     }));
 
     test('can log and dump versions & tokens to a file', overridePrint(() {
@@ -369,3 +428,22 @@ static final String tokenFoo = '${getToken('foo')}';
 static final String tokenBar = '${getToken('bar')}';
 ''';
 }
+
+class TestColorTemplate extends TokenTemplate {
+  TestColorTemplate(super.blockName, super.fileName, super.tokens);
+
+  @override
+  String generate() => '''
+static final Color color_1 = '${color('color_foo_req')}';
+static final Color color_2 = '${color('color_foo_opt', 'Colors.red')}';
+''';
+}
+
+class TestBorderTemplate extends TokenTemplate {
+  TestBorderTemplate(super.blockName, super.fileName, super.tokens);
+
+  @override
+  String generate() => '''
+static final BorderSide border = '${border('border_foo')}';
+''';
+}