Handle dollar signs properly when generating localizations (#125514)

Currently, the code doesn't properly handle strings which contain dollar signs. The return expression for the generated localization function is computed by `generateReturnExpr` which concatenates several strings, which are either interpolated placeholders, interpolated function calls, or normal strings, but we didn't properly escape dollar signs before sending normal strings to `generateReturnExpr`.

Fixes #125461.

packages/flutter_tools/lib/src/localizations/gen_l10n.dart

@@ -1131,7 +1131,7 @@ class LocalizationsGenerator {
           case ST.message:
             final List<String> expressions = node.children.map<String>((Node node) {
               if (node.type == ST.string) {
-                return node.value!;
+                return generateString(node.value!);
               }
               return generateVariables(node);
             }).toList();

packages/flutter_tools/lib/src/localizations/localizations_utils.dart

@@ -291,10 +291,25 @@ String generateString(String value) {
   return value;
 }
 
-/// Given a list of strings, placeholders, or helper function calls, concatenate
-/// them into one expression to be returned.
+/// Given a list of normal strings or interpolated variables, concatenate them
+/// into a single dart string to be returned. An example of a normal string
+/// would be "'Hello world!'" and an example of a interpolated variable would be
+/// "'$placeholder'".
 ///
-/// If `isSingleStringVar` is passed, then we want to convert "'$expr'" to "expr".
+/// Each of the strings in [expressions] should be a raw string, which, if it
+/// were to be added to a dart file, would be a properly formatted dart string
+/// with escapes and/or interpolation. The purpose of this function is to
+/// concatenate these dart strings into a single dart string which can be
+/// returned in the generated localization files.
+///
+/// The following rules describe the kinds of string expressions that can be
+/// handled:
+/// 1. If [expressions] is empty, return the empty string "''".
+/// 2. If [expressions] has only one [String] which is an interpolated variable,
+///    it is converted to the variable itself e.g. ["'$expr'"] -> "expr".
+/// 3. If one string in [expressions] is an interpolation and the next begins
+///    with an alphanumeric character, then the former interpolation should be
+///    wrapped in braces e.g. ["'$expr1'", "'another'"] -> "'${expr1}another'".
 String generateReturnExpr(List<String> expressions, { bool isSingleStringVar = false }) {
   if (expressions.isEmpty) {
     return "''";
@@ -304,7 +319,7 @@ String generateReturnExpr(List<String> expressions, { bool isSingleStringVar = f
   } else {
     final String string = expressions.reversed.fold<String>('', (String string, String expression) {
       if (expression[0] != r'$') {
-        return generateString(expression) + string;
+        return expression + string;
       }
       final RegExp alphanumeric = RegExp(r'^([0-9a-zA-Z]|_)+$');
       if (alphanumeric.hasMatch(expression.substring(1)) && !(string.isNotEmpty && alphanumeric.hasMatch(string[0]))) {

packages/flutter_tools/test/general.shard/generate_localizations_test.dart

@@ -3244,4 +3244,32 @@ NumberFormat.decimalPatternDigits(
 );
 '''));
   });
+
+  // Regression test for https://github.com/flutter/flutter/issues/125461.
+  testWithoutContext('dollar signs are escaped properly when there is a select clause', () {
+    const String dollarSignWithSelect = r'''
+{
+  "dollarSignWithSelect": "$nice_bug\nHello Bug! Manifistation #1 {selectPlaceholder, select, case{message} other{messageOther}}"
+}''';
+    final Directory l10nDirectory = fs.currentDirectory.childDirectory('lib').childDirectory('l10n')
+      ..createSync(recursive: true);
+    l10nDirectory.childFile(defaultTemplateArbFileName)
+      .writeAsStringSync(dollarSignWithSelect);
+    LocalizationsGenerator(
+      fileSystem: fs,
+      inputPathString: defaultL10nPathString,
+      outputPathString: defaultL10nPathString,
+      templateArbFileName: defaultTemplateArbFileName,
+      outputFileString: defaultOutputFileString,
+      classNameString: defaultClassNameString,
+      logger: logger,
+      suppressWarnings: true,
+    )
+      ..loadResources()
+      ..writeOutputFiles();
+    final String localizationsFile = fs.file(
+      fs.path.join(syntheticL10nPackagePath, 'output-localization-file_en.dart'),
+    ).readAsStringSync();
+    expect(localizationsFile, contains(r'\$nice_bug\nHello Bug! Manifistation #1 $_temp0'));
+  });
 }