Introduce getPrefixEndToken to avoid too big context in messages.

pkg/compiler/lib/src/diagnostics/source_span.dart

 // Copyright (c) 2012, 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.diagnostics.source_span;
 
 import '../tokens/token.dart' show
     Token;
 import '../tree/tree.dart' show
     Node;
 import 'spannable.dart' show
     Spannable;
 
 class SourceSpan implements Spannable {
   final Uri uri;
   final int begin;
   final int end;
 
   const SourceSpan(this.uri, this.begin, this.end);
 
   factory SourceSpan.fromNode(Uri uri, Node node) {
     return new SourceSpan.fromTokens(
-        uri, node.getBeginToken(), node.getEndToken());
+        uri, node.getBeginToken(), node.getPrefixEndToken());
   }
 
   factory SourceSpan.fromTokens(Uri uri, Token begin, Token end) {
     final beginOffset = begin.charOffset;
     final endOffset = end.charOffset + end.charCount;
 
     // [begin] and [end] might be the same for the same empty token. This
     // happens for instance when scanning '$$'.
     assert(endOffset >= beginOffset);
     return new SourceSpan(uri, beginOffset, endOffset);
   }
 
   int get hashCode {
     return 13 * uri.hashCode +
            17 * begin.hashCode +
            19 * end.hashCode;
   }
 
   bool operator ==(other) {
     if (identical(this, other)) return true;
     if (other is! SourceSpan) return false;
     return uri == other.uri &&
            begin == other.begin &&
            end == other.end;
   }
 
   String toString() => 'SourceSpan($uri, $begin, $end)';
 }