Fix for resynthesizing libraries with invalid part URIs.

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

Index: pkg/analyzer/test/src/summary/resynthesize_ast_test.dart
diff --git a/pkg/analyzer/test/src/summary/resynthesize_ast_test.dart b/pkg/analyzer/test/src/summary/resynthesize_ast_test.dart
index 29e30d3860b49dd7ba98c81ef0ce439837a179e5..aa9cde3d41c5cf0c6cc413d6a90c8a423eb4d0ac 100644
--- a/pkg/analyzer/test/src/summary/resynthesize_ast_test.dart
+++ b/pkg/analyzer/test/src/summary/resynthesize_ast_test.dart
@@ -832,6 +832,10 @@ abstract class _AstResynthesizeTestMixin
   }
 
   UnlinkedUnit _getUnlinkedUnit(Source source) {
+    if (source == null) {
+      return new UnlinkedUnitBuilder();
+    }
+
     String uriStr = source.uri.toString();
     {
       UnlinkedUnit unlinkedUnitInSdk =
@@ -873,7 +877,7 @@ abstract class _AstResynthesizeTestMixin
     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})');
       }