Report error on only-try block.

pkg/front_end/tool/_fasta/generate_messages.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:io';
 
 import 'dart:isolate';
 
 import 'package:yaml/yaml.dart' show loadYaml;
 
@@ skipping 46 matching lines @@
   await new File.fromUri(problemsFile).writeAsString("$sb", flush: true);
   port.close();
 }
 
 final RegExp placeholderPattern = new RegExp("#[a-zA-Z0-9_]+");
 
 String compileTemplate(String name, String template, String tip) {
   var parameters = new Set<String>();
   var conversions = new Set<String>();
   var arguments = new Set<String>();
-  for (Match match in placeholderPattern.allMatches("$template$tip")) {
+  for (Match match in placeholderPattern.allMatches("$template${tip ?? ''}")) {
     switch (match[0]) {
       case "#character":
         parameters.add("String character");
         arguments.add("'character': character,");
         break;
 
       case "#unicode":
         parameters.add("int codePoint");
         conversions.add("String unicode = "
             "\"(U+\${codePoint.toRadixString(16).padLeft(4, '0')})\";");
         arguments.add("'codePoint': codePoint,");
         break;
 
       case "#name":
         parameters.add("String name");
         arguments.add("'name': name,");
         break;
 
       case "#lexeme":
         parameters.add("Token token");
         conversions.add("String lexeme = token.lexeme;");
         arguments.add("'token': token,");
         break;
 
       case "#string":
         parameters.add("String string");
         arguments.add("'string': string,");
         break;
+
+      default:
+        throw "Unhandled placeholder in template: ${match[0]}";
     }
   }
 
   String interpolate(String name, String text) {
     if (text == null) return "";
     return "  '$name': "
         "\"${text.replaceAll(r'$', r'\$').replaceAll('#', '\$')}\",";
   }
 
   return """
 problem$name(${parameters.join(', ')}) {
   // DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
   ${conversions.join('\n  ')}
   return {
   ${interpolate('message', template)}
   ${interpolate('tip', tip)}
     'code': ErrorKind.$name,
     'arguments': {
       ${arguments.join('\n      ')}
     },
   };
 }
 """;
 }