Ensure that nulls aren't added to constant eval dependency graph.

pkg/analyzer/lib/src/generated/constant.dart

Index: pkg/analyzer/lib/src/generated/constant.dart
diff --git a/pkg/analyzer/lib/src/generated/constant.dart b/pkg/analyzer/lib/src/generated/constant.dart
index effe860933982c5f07ee1a8b79d48ec8cd3bca94..b3d41078949be1f47eb86447d0b22bb13112331f 100644
--- a/pkg/analyzer/lib/src/generated/constant.dart
+++ b/pkg/analyzer/lib/src/generated/constant.dart
@@ -1198,6 +1198,7 @@ class ConstantValueComputer {
       }
     } else {
       // Should not happen.
+      assert(false);
       AnalysisEngine.instance.logger.logError(
           "Constant value computer trying to compute the value of a node which is not a VariableDeclaration, InstanceCreationExpression, FormalParameter, or ConstructorDeclaration");
       return;
@@ -4921,7 +4922,9 @@ class ReferenceFinder extends RecursiveAstVisitor<Object> {
       if (constructor != null) {
         ConstructorDeclaration declaration =
             findConstructorDeclaration(constructor);
-        _referenceGraph.addEdge(_source, declaration);
+        if (declaration != null) {
+          _referenceGraph.addEdge(_source, declaration);
+        }
       }
     }
     return super.visitInstanceCreationExpression(node);