Add support for external summaries bundle in AnalysisDriver.

pkg/analyzer/lib/src/dart/analysis/library_analyzer.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/context/declared_variables.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/error/error.dart';
 import 'package:analyzer/error/listener.dart';
@@ skipping 740 matching lines @@
     throw new UnimplementedError();
   }
 
   @override
   String getContents(Source source) {
     return _getFileForSource(source).content;
   }
 
   @override
   bool getExists(Source source) {
+    if (fsState.externalSummaries != null) {
+      String uriStr = source.uri.toString();
+      if (fsState.externalSummaries.hasUnlinkedUnit(uriStr)) {
+        return true;
+      }
+    }
     return _getFileForSource(source).exists;
   }
 
   @override
   int getModificationStamp(Source source) {
     return _getFileForSource(source).exists ? 0 : -1;
   }
 
   @override
   String setContents(Source source, String contents) {
     throw new UnimplementedError();
   }
 
   FileState _getFileForSource(Source source) {
     String path = source.fullName;
     return fsState.getFileForPath(path);
   }
 }
 
 /**
  * Either the name or the source associated with a part-of directive.
  */
 class _NameOrSource {
   final String name;
   final Source source;
   _NameOrSource(this.name, this.source);
 }