Introduce getPrefixEndToken to avoid too big context in messages.

tests/compiler/dart2js/diagnostic_helper.dart

 // Copyright (c) 2015, 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 dart2js.test.diagnostic_helper;
 
 import 'dart:collection';
 
 import 'package:compiler/compiler_new.dart' show
     CompilerDiagnostics,
     Diagnostic;
 import 'package:compiler/src/diagnostics/messages.dart' show
     Message,
     MessageKind;
 import 'package:expect/expect.dart';
 
 class CollectedMessage {
   final Message message;
   final Uri uri;
   final int begin;
   final int end;
   final String text;
   final Diagnostic kind;
 
   CollectedMessage(
       this.message, this.uri, this.begin, this.end, this.text, this.kind);
 
-  MessageKind get messageKind => message.kind;
+  MessageKind get messageKind => message?.kind;
 
   String toString() {
     return '${message != null ? message.kind : ''}'
            ':$uri:$begin:$end:$text:$kind';
   }
 }
 
 class DiagnosticCollector implements CompilerDiagnostics {
   List<CollectedMessage> messages = <CollectedMessage>[];
 
@@ skipping 146 matching lines @@
       String expectedValue = '${arguments[key]}';
       String foundValue = '${message.arguments[key]}';
       if (expectedValue != foundValue) {
         return 'Expected argument $key with value $expectedValue, '
                'found $foundValue.';
       }
     }
     return null;
   };
 }