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

pkg/analyzer/test/src/context/context_test.dart

 // Copyright (c) 2014, 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.test.src.context.context_test;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 1379 matching lines @@
     _analyzeAll_assertFinished();
     expect(context.getResolvedCompilationUnit2(source, source), isNotNull);
     // new contents
     contentCache.setContents(source, newContents);
     context.handleContentsChanged(source, oldContents, newContents, true);
     // there is some work to do
     AnalysisResult analysisResult = context.performAnalysisTask();
     expect(analysisResult.changeNotices, isNotNull);
   }
 
+  void test_handleContentsChanged_incremental_newContentsNull() {
+    context.analysisOptions = new AnalysisOptionsImpl()..incremental = true;
+    ContentCache contentCache = new ContentCache();
+    context.contentCache = contentCache;
+    // old contents
+    String oldContents = 'foo() {}';
+    Source source = resourceProvider.getFile('/test.dart').createSource();
+    contentCache.setContents(source, oldContents);
+    expect(context.computeLibraryElement(source), isNotNull);
+    // new contents
+    String newContents = null;
+    contentCache.setContents(source, newContents);
+    context.handleContentsChanged(source, oldContents, newContents, true);
+    expect(context.getLibraryElement(source), isNull);
+  }
+
   Future test_implicitAnalysisEvents_added() async {
     AnalyzedSourcesListener listener = new AnalyzedSourcesListener();
     context.implicitAnalysisEvents.listen(listener.onData);
     //
     // Create a file that references an file that is not explicitly being
     // analyzed and fully analyze it. Ensure that the listener is told about
     // the implicitly analyzed file.
     //
     Source sourceA = newSource('/a.dart', "library a; import 'b.dart';");
     Source sourceB = newSource('/b.dart', "library b;");
@@ skipping 1516 matching lines @@
    * Initialize the visitor.
    */
   _ElementGatherer();
 
   @override
   void visitElement(Element element) {
     elements[element] = element;
     super.visitElement(element);
   }
 }