Return empty contents for not existing sources.

pkg/analyzer/lib/src/context/cache.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.context.cache;
 
 import 'dart:collection';
 
 import 'package:analyzer/src/generated/engine.dart'
     show AnalysisEngine, CacheState, InternalAnalysisContext, RetentionPriority;
@@ skipping 207 matching lines @@
    */
   final AnalysisTarget target;
 
   /**
    * The partition that is responsible for this entry.
    */
   CachePartition _partition;
 
   /**
    * The most recent time at which the state of the target matched the state
-   * represented by this entry.
+   * represented by this entry, `-1` if the target does not exist.
    */
-  int modificationTime = 0;
+  int modificationTime = -1;
 
   /**
    * The exception that caused one or more values to have a state of
    * [CacheState.ERROR].
    */
   CaughtException _exception;
 
   /**
    * A bit-encoding of boolean flags associated with this entry's target.
    */
@@ skipping 691 matching lines @@
 class UniversalCachePartition extends CachePartition {
   /**
    * Initialize a newly created cache partition, belonging to the given
    * [context].
    */
   UniversalCachePartition(InternalAnalysisContext context) : super(context);
 
   @override
   bool isResponsibleFor(AnalysisTarget target) => true;
 }