Fix for explicit dart:core import.

pkg/analyzer/lib/src/task/dart.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 analyzer.src.task.dart;
 
 import 'dart:collection';
 import 'dart:math' as math;
 
 import 'package:analyzer/src/context/cache.dart';
@@ skipping 841 matching lines @@
         String uriContent = importDirective.uriContent;
         if (DartUriResolver.isDartExtUri(uriContent)) {
           libraryElement.hasExtUri = true;
         }
         Source importedSource = importDirective.source;
         if (importedSource != null && context.exists(importedSource)) {
           // The imported source will be null if the URI in the import
           // directive was invalid.
           LibraryElement importedLibrary = importLibraryMap[importedSource];
           if (importedLibrary != null) {
+            if (importedLibrary.isDartCore) {
+              explicitlyImportsCore = true;
+            }
             ImportElementImpl importElement =
                 new ImportElementImpl(directive.offset);
             StringLiteral uriLiteral = importDirective.uri;
             if (uriLiteral != null) {
               importElement.uriOffset = uriLiteral.offset;
               importElement.uriEnd = uriLiteral.end;
             }
             importElement.uri = uriContent;
             importElement.deferred = importDirective.deferredKeyword != null;
             importElement.combinators = _buildCombinators(importDirective);
@@ skipping 2652 matching lines @@
   @override
   bool moveNext() {
     if (_newSources.isEmpty) {
       return false;
     }
     currentTarget = _newSources.first;
     _newSources.remove(currentTarget);
     return true;
   }
 }