gracefully recover from extraneous top level modifiers

pkg/front_end/messages.yaml

 # 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.
 
 # Each entry in this map corresponds to a diagnostic message. Ideally, each
 # entry contains three parts:
 #
 # 1. A message template (template).
 #
 # 2. A suggestion for how to correct the problem (tip).
@@ skipping 183 matching lines @@
     - "var var foo; main(){}"
     - "const var foo; main(){}"
     - "abstract var foo; main(){}"
     - "static var foo; main(){}"
     - "external var foo; main(){}"
     - "set foo; main(){}"
     - "abstract foo; main(){}"
     - "static foo; main(){}"
     - "external foo; main(){}"
     - "class C {abstract C.c();}"
+    - "final class C {}"
+    - "abstract enum foo {bar}"
+    - "const enum foo {bar}"
+    - "final enum foo {bar}"
+    - "abstract void foo() {}"
+    - "static void foo() {}"
+    - "abstract typedef foo();"
+    - "const typedef foo();"
+    - "final typedef foo();"
+    - "static typedef foo();"
 
 InvalidAwaitFor:
   template: "'await' is only supported in methods with an 'async' or 'async*' body modifier."
   tip: "Try adding 'async' or 'async*' to the method body or removing the 'await' keyword."
   dart2jsCode: INVALID_AWAIT_FOR
   script: >
     main(o) sync* {
       await for (var e in o) {}
     }
 
@@ skipping 697 matching lines @@
 PackageNotFound:
   template: "Could not resolve the package '#name' in '#uri'."
 
 InvalidPackageUri:
   template: "Invalid package Uri '#uri':\n  #string."
 
 IntegerLiteralIsOutOfRange:
   template: "The integer literal #lexeme can't be represented in 64 bits."
   tip: "Try using BigInt (from 'dart:typed_data' library) if you need an integer larger than 9,223,372,036,854,775,807 or less than -9,223,372,036,854,775,808."