Add implicit dart:core import when resynthsize from Kernel.

pkg/analyzer/lib/src/kernel/resynthesize.dart

 // Copyright (c) 2017, 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.
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_ast_factory.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/dart/element/element.dart';
@@ skipping 474 matching lines @@
 
   @override
   final kernel.Library library;
 
   Source librarySource;
   LibraryElementImpl libraryElement;
 
   _KernelLibraryResynthesizerContextImpl(this.resynthesizer, this.library);
 
   @override
+  kernel.Library get coreLibrary => resynthesizer._kernelMap['dart:core'];
+
+  @override
   LibraryElementImpl getLibrary(String uriStr) {
     return resynthesizer.getLibrary(uriStr);
   }
 
   LibraryElementImpl _buildLibrary(String uriStr) {
     librarySource = resynthesizer._getSource(uriStr);
     return libraryElement =
         new LibraryElementImpl.forKernel(resynthesizer._analysisContext, this);
   }
 
@@ skipping 310 matching lines @@
         for (var typeParameter in ctx.typeParameters) {
           if (typeParameter.name == name) {
             return typeParameter;
           }
         }
       }
     }
     throw new StateError('Not found $kernelTypeParameter in $context');
   }
 }