Better error message for missing async with await

pkg/analyzer/lib/src/error/codes.dart

 // Copyright (c) 2014, 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.
 
 library analyzer.src.error.codes;
 
 import 'package:analyzer/error/error.dart';
 
 export 'package:analyzer/src/analysis_options/error/option_codes.dart';
 export 'package:analyzer/src/dart/error/hint_codes.dart';
@@ skipping 4524 matching lines @@
       "Undefined name '{0}'.",
       "Try correcting the name to one that is defined, or "
       "defining the name.");
 
   /**
    * If the identifier is 'await', be helpful about it.
    */
   static const StaticWarningCode UNDEFINED_IDENTIFIER_AWAIT =
       const StaticWarningCode(
           'UNDEFINED_IDENTIFIER_AWAIT',
-          "Undefined name 'await'.",
+          "Undefined name 'await' in function body not marked with 'async'.",
           "Try correcting the name to one that is defined, "
           "defining the name, or "
           "adding 'async' to the enclosing function body.");
 
   /**
    * 12.14.2 Binding Actuals to Formals: Furthermore, each <i>q<sub>i</sub></i>,
    * <i>1<=i<=l</i>, must have a corresponding named parameter in the set
    * {<i>p<sub>n+1</sub></i> &hellip; <i>p<sub>n+k</sub></i>} or a static
    * warning occurs.
    *
@@ skipping 371 matching lines @@
    * created from the optional [correction] template.
    */
   const StrongModeCode(ErrorType type, String name, String message,
       [String correction])
       : type = type,
         super('STRONG_MODE_$name', message, correction);
 
   @override
   ErrorSeverity get errorSeverity => type.severity;
 }