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

lib/src/dependency_graph.dart

Index: lib/src/dependency_graph.dart
diff --git a/lib/src/dependency_graph.dart b/lib/src/dependency_graph.dart
index 817a9c073a225aebff2ecf5e61a9d956f8f7a9e0..5d32d72116ecfb15144802a26e6c5b701cb2f5ed 100644
--- a/lib/src/dependency_graph.dart
+++ b/lib/src/dependency_graph.dart
@@ -97,7 +97,9 @@ abstract class SourceNode {
   /// 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.