Fix for resynthesizing libraries with invalid part URIs.

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 814 matching lines @@
         new Map<String, UnlinkedUnit>()
           ..addAll(SerializedMockSdk.instance.uriToUnlinkedUnit)
           ..addAll(unlinkedSummaries),
         new Map<String, LinkedLibrary>()
           ..addAll(SerializedMockSdk.instance.uriToLinkedLibrary)
           ..addAll(linkedSummaries),
         allowMissingFiles);
   }
 
   UnlinkedUnit _getUnlinkedUnit(Source source) {
+    if (source == null) {
+      return new UnlinkedUnitBuilder();
+    }
+
     String uriStr = source.uri.toString();
     {
       UnlinkedUnit unlinkedUnitInSdk =
           SerializedMockSdk.instance.uriToUnlinkedUnit[uriStr];
       if (unlinkedUnitInSdk != null) {
         return unlinkedUnitInSdk;
       }
     }
     return uriToUnit.putIfAbsent(uriStr, () {
       int modificationTime = context.computeResult(source, MODIFICATION_TIME);
@@ skipping 21 matching lines @@
     if (librarySource.isInSystemLibrary) {
       return;
     }
     if (!serializedSources.add(librarySource)) {
       return;
     }
 
     Source resolveRelativeUri(String relativeUri) {
       Source resolvedSource =
           context.sourceFactory.resolveUri(librarySource, relativeUri);
-      if (resolvedSource == null) {
+      if (resolvedSource == null && !allowMissingFiles) {
         throw new StateError('Could not resolve $relativeUri in the context of '
             '$librarySource (${librarySource.runtimeType})');
       }
       return resolvedSource;
     }
 
     UnlinkedUnit getPart(String relativeUri) {
       return _getUnlinkedUnit(resolveRelativeUri(relativeUri));
     }
 
@@ skipping 52 matching lines @@
   }
 
   @override
   DartSdk createDartSdk() => AbstractContextTest.SHARED_MOCK_SDK;
 
   @override
   TestSummaryResynthesizer encodeDecodeLibrarySource(Source source) {
     return _encodeLibrary(source);
   }
 }