Fix the value of multiline strings

pkg/analyzer/lib/src/generated/ast.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.
 
 // This code was auto-generated, is not intended to be edited, and is subject to
 // significant change. Please see the README file for more information.
 
 library engine.ast;
 
 import 'dart:collection';
@@ skipping 16450 matching lines @@
   }
 
   @override
   Token get beginToken => literal;
 
   @override
   Iterable get childEntities => new ChildEntities()..add(literal);
 
   @override
   int get contentsEnd {
-    return contentsOffset + value.length;
+    if (isMultiline) {
+      return end - 3;
+    }
+    return end - 1;
   }
 
   @override
   int get contentsOffset {
     int contentsOffset = 0;
     if (isRaw) {
       contentsOffset += 1;
     }
     if (isMultiline) {
       contentsOffset += 3;
@@ skipping 3576 matching lines @@
   }
 
   @override
   accept(AstVisitor visitor) => visitor.visitYieldStatement(this);
 
   @override
   void visitChildren(AstVisitor visitor) {
     _safelyVisitChild(_expression, visitor);
   }
 }