Perform incremental resolution only if the existing unit is not null.

pkg/analyzer/lib/src/generated/engine.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.
 
 // This code was auto-generated, is not intended to be edited, and is subject to
 // significant change. Please see the README file for more information.
 
 library engine;
 
 import "dart:math" as math;
@@ skipping 5073 matching lines @@
   bool _tryPoorMansIncrementalResolution(Source unitSource, String newCode) {
     DartEntry dartEntry = _cache.get(unitSource);
     if (dartEntry == null) {
       return false;
     }
     // prepare the (only) library
     List<Source> librarySources = getLibrariesContaining(unitSource);
     if (librarySources.length != 1) {
       return false;
     }
-    // do resolution
+    // prepare the existing unit
     Source librarySource = librarySources[0];
     CompilationUnit oldUnit =
         getResolvedCompilationUnit2(unitSource, librarySource);
+    if (oldUnit == null) {
+      return false;
+    }
+    // do resolution
     PoorMansIncrementalResolver resolver = new PoorMansIncrementalResolver(
         typeProvider,
         unitSource,
         dartEntry,
         analysisOptions.incrementalApi);
     bool success = resolver.resolve(oldUnit, newCode);
     if (!success) {
       return false;
     }
     // prepare notice
     ChangeNoticeImpl notice = _getNotice(unitSource);
     notice.compilationUnit = oldUnit;
-    // TODO(scheglov) apply updated errors
+    // apply updated errors
     {
       LineInfo lineInfo = getLineInfo(unitSource);
       notice.setErrors(dartEntry.allErrors, lineInfo);
     }
     return true;
   }
 
   /**
    * Check the cache for any invalid entries (entries whose modification time does not match the
    * modification time of the source associated with the entry). Invalid entries will be marked as
@@ skipping 9302 matching lines @@
 
   @override
   Object visitPolymerTagDartElement(PolymerTagDartElement element) {
     if (element.name == PolymerHtmlUnitBuilder_this._elementName) {
       throw new PolymerHtmlUnitBuilder_FoundTagDartElementError(
           element as PolymerTagDartElementImpl);
     }
     return null;
   }
 }