Issue 26106. Don't attempt to incrementally resolve sources without contents.

pkg/analyzer/lib/src/context/context.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.context;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 1048 matching lines @@
         newContents = fileContents.data;
         entry.modificationTime = fileContents.modificationTime;
         if (newContents == originalContents) {
           entry.setValue(CONTENT, newContents, TargetedResult.EMPTY_LIST);
           changed = false;
         }
       } catch (e) {}
       // If not the same content (e.g. the file is being closed without save),
       // then force analysis.
       if (changed) {
-        if (!analysisOptions.incremental ||
+        if (newContents == null ||
+            !analysisOptions.incremental ||
             !_tryPoorMansIncrementalResolution(source, newContents)) {
           _sourceChanged(source);
         }
       }
     }
     if (notify && changed) {
       _onSourcesChangedController
           .add(new SourcesChangedEvent.changedContent(source, newContents));
     }
     return changed;
@@ skipping 1116 matching lines @@
     }
     DartSdk sdk = factory.dartSdk;
     if (sdk == null) {
       throw new IllegalArgumentException(
           "The source factory for an SDK analysis context must have a DartUriResolver");
     }
     return new AnalysisCache(
         <CachePartition>[AnalysisEngine.instance.partitionManager.forSdk(sdk)]);
   }
 }