Handle invalid URIs in summaries.

pkg/analyzer/test/src/summary/resynthesize_ast_test.dart

 // Copyright (c) 2016, 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.summary.resynthesize_ast_test;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/error/error.dart';
 import 'package:analyzer/src/dart/element/element.dart';
@@ skipping 840 matching lines @@
     });
   }
 
   void _reset() {
     serializedSources.clear();
     bundleAssembler = new PackageBundleAssembler();
     uriToUnit.clear();
   }
 
   void _serializeLibrary(Source librarySource) {
-    if (librarySource.isInSystemLibrary) {
+    if (librarySource == null || librarySource.isInSystemLibrary) {
       return;
     }
     if (!serializedSources.add(librarySource)) {
       return;
     }
 
     Source resolveRelativeUri(String relativeUri) {
       Source resolvedSource =
           context.sourceFactory.resolveUri(librarySource, relativeUri);
       if (resolvedSource == null && !allowMissingFiles) {
@@ skipping 62 matching lines @@
   }
 
   @override
   DartSdk createDartSdk() => AbstractContextTest.SHARED_MOCK_SDK;
 
   @override
   TestSummaryResynthesizer encodeDecodeLibrarySource(Source source) {
     return _encodeLibrary(source);
   }
 }