Fix co19 crashes

dart/frog/leg/scanner/scanner_task.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.
 
 class ScannerTask extends CompilerTask {
   ScannerTask(Compiler compiler) : super(compiler);
   String get name() => 'Scanner';
 
   void scan(CompilationUnitElement compilationUnit) {
     measure(() {
@@ skipping 92 matching lines @@
     return library;
   }
 
   void importLibrary(LibraryElement library, LibraryElement imported,
                      ScriptTag tag) {
     if (tag !== null && tag.prefix !== null) {
       SourceString prefix =
           new SourceString(tag.prefix.dartString.slowToString());
       Element e = library.find(prefix);
       if (e === null) {
-        e = new PrefixElement(prefix, library);
+        e = new PrefixElement(prefix, library, tag.getBeginToken());
         library.define(e, compiler);
       }
       if (e.kind !== ElementKind.PREFIX) {
         compiler.withCurrentElement(e, () {
           compiler.reportWarning(new Identifier(e.position()),
                                  'duplicated definition');
         });
         compiler.reportError(tag.prefix, 'duplicate defintion');
       }
       imported.forEachExport((Element element) {
@@ skipping 24 matching lines @@
   final String name = 'Diet Parser';
 
   dietParse(CompilationUnitElement compilationUnit, Token tokens) {
     measure(() {
       ElementListener listener = new ElementListener(compiler, compilationUnit);
       PartialParser parser = new PartialParser(listener);
       parser.parseUnit(tokens);
     });
   }
 }