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

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

Index: pkg/analyzer/test/src/context/context_test.dart
diff --git a/pkg/analyzer/test/src/context/context_test.dart b/pkg/analyzer/test/src/context/context_test.dart
index 63130a5cbcfd905f9872605145a5105d4ff7d183..b2e1f0493d69518eaa94363a97cbbbadfc6ecb5e 100644
--- a/pkg/analyzer/test/src/context/context_test.dart
+++ b/pkg/analyzer/test/src/context/context_test.dart
@@ -1397,6 +1397,22 @@ main() {}''');
     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);