Fix in multi-package-resolver to support files that will be created later (graph

lib/src/dependency_graph.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.
 
 /// Tracks the shape of the import/export graph and dependencies between files.
 library dev_compiler.src.dependency_graph;
 
 import 'dart:collection' show HashSet;
 
 import 'package:analyzer/analyzer.dart' show parseDirectives;
@@ skipping 79 matching lines @@
 
   /// Direct dependencies in the [SourceGraph]. These include script tags for
   /// [HtmlSourceNode]s; and imports, exports and parts for [DartSourceNode]s.
   Iterable<SourceNode> get allDeps => const [];
 
   /// Like [allDeps] but excludes parts for [DartSourceNode]s. For many
   /// operations we mainly care about dependencies at the library level, so
   /// parts are excluded from this list.
   Iterable<SourceNode> get depsWithoutParts => const [];
 
-  SourceNode(this.uri, this.source);
+  SourceNode(this.uri, this.source) {
+    assert(source != null);
+  }
 
   /// Check for whether the file has changed and, if so, mark [needsRebuild] and
   /// [structureChanged] as necessary.
   void update(SourceGraph graph) {
     int newStamp = source.modificationStamp;
     if (newStamp > _lastStamp) {
       _lastStamp = newStamp;
       needsRebuild = true;
     }
   }
@@ skipping 297 matching lines @@
   helper(start);
 }
 
 bool _same(Set a, Set b) => a.length == b.length && a.containsAll(b);
 
 /// An error message discovered while parsing the dependencies between files.
 class DependencyGraphError extends MessageWithSpan {
   const DependencyGraphError(String message, SourceSpan span)
       : super(message, Level.SEVERE, span);
 }