Use messages for (some) public API errors

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 33 matching lines @@
       Uri.parse('package:front_end/src/fasta/fasta_codes_generated.dart'));
   await new File.fromUri(problemsFile)
       .writeAsString(dartfmtedText, flush: true);
   port.close();
 }
 
 final RegExp placeholderPattern = new RegExp("#[a-zA-Z0-9_]+");
 
 String compileTemplate(String name, String template, String tip,
     String analyzerCode, String dart2jsCode) {
+  if (template == null) {
+    print('Error: missing template for message: $name');
+    exit(1);
+    return '';
+  }
   var parameters = new Set<String>();
   var conversions = new Set<String>();
   var arguments = new Set<String>();
   for (Match match in placeholderPattern.allMatches("$template${tip ?? ''}")) {
     switch (match[0]) {
       case "#character":
         parameters.add("String character");
         arguments.add("'character': character");
         break;
 
@@ skipping 86 matching lines @@
 
 // DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
 Message _withArguments$name(${parameters.join(', ')}) {
   ${conversions.join('\n  ')}
   return new Message(
      code$name,
      ${messageArguments.join(', ')});
 }
 """;
 }