Introduce getPrefixEndToken to avoid too big context in messages.

pkg/compiler/lib/src/elements/modelx.dart

 // Copyright (c) 2013, 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 elements.modelx;
 
 import '../common.dart';
 import '../common/resolution.dart' show
     Resolution,
     Parsing;
@@ skipping 2097 matching lines @@
       : super(name, kind, modifiers, enclosing);
 
   MemberElement get memberContext => this;
 
   @override
   SourceSpan get sourcePosition {
     SourceSpan span = super.sourcePosition;
     if (span != null && hasNode) {
       FunctionExpression functionExpression = node.asFunctionExpression();
       if (functionExpression != null) {
-        Token begin = functionExpression.getBeginToken();
-        Token end;
-        if (functionExpression.parameters != null) {
-          end = functionExpression.parameters.getEndToken();
-        } else {
-          end = functionExpression.name.getEndToken();
-        }
-        span = new SourceSpan.fromTokens(span.uri, begin, end);
+        span = new SourceSpan.fromNode(span.uri, functionExpression);
       }
     }
     return span;
   }
 
   void reuseElement() {
     super.reuseElement();
     nestedClosures.clear();
     _functionSignatureCache = null;
     typeCache = null;
@@ skipping 1092 matching lines @@
   AstElement get definingElement;
 
   bool get hasResolvedAst => definingElement.hasTreeElements;
 
   ResolvedAst get resolvedAst {
     return new ResolvedAst(declaration,
         definingElement.node, definingElement.treeElements);
   }
 
 }