Ignore not existing parts.

pkg/analyzer/lib/src/task/dart.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.task.dart;
 
 import 'dart:collection';
 import 'dart:math' as math;
 
 import 'package:analyzer/src/context/cache.dart';
@@ skipping 1304 matching lines @@
         if (libraryNameNode == null) {
           libraryNameNode = directive.name;
           directivesToResolve.add(directive);
         }
       } else if (directive is PartDirective) {
         PartDirective partDirective = directive;
         StringLiteral partUri = partDirective.uri;
         Source partSource = partDirective.source;
         hasPartDirective = true;
         CompilationUnit partUnit = partUnitMap[partSource];
-        if (partUnit != null) {
+        if (partUnit != null && context.exists(partSource)) {
           CompilationUnitElementImpl partElement = partUnit.element;
           partElement.uriOffset = partUri.offset;
           partElement.uriEnd = partUri.end;
           partElement.uri = partDirective.uriContent;
           //
           // Validate that the part contains a part-of directive with the same
           // name as the library.
           //
           String partLibraryName =
               _getPartLibraryName(partSource, partUnit, directivesToResolve);
@@ skipping 2028 matching lines @@
   @override
   bool moveNext() {
     if (_newSources.isEmpty) {
       return false;
     }
     currentTarget = _newSources.first;
     _newSources.remove(currentTarget);
     return true;
   }
 }