Patch to work around exception until the root cause is found

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

Index: pkg/analyzer/lib/src/dart/constant/evaluation.dart
diff --git a/pkg/analyzer/lib/src/dart/constant/evaluation.dart b/pkg/analyzer/lib/src/dart/constant/evaluation.dart
index da2db5551640abb588293d10c67fc6c99ec14154..30d69951d061ce4012478e844ba134ebe28f6d8c 100644
--- a/pkg/analyzer/lib/src/dart/constant/evaluation.dart
+++ b/pkg/analyzer/lib/src/dart/constant/evaluation.dart
@@ -347,7 +347,13 @@ class ConstantEvaluationEngine {
         // This could happen in the event of invalid code.  The error will be
         // reported at constant evaluation time.
       }
-      if (constNode.arguments != null) {
+      if (constNode == null) {
+        // We cannot determine what element the annotation is on, nor the offset
+        // of the annotation, so there's not a lot of information in this
+        // message, but it's better than getting an exception.
+        AnalysisEngine.instance.logger.logInformation(
+            'No annotationAst for $constant in ${constant.compilationUnit}');
+      } else if (constNode.arguments != null) {
         constNode.arguments.accept(referenceFinder);
       }
     } else if (constant is VariableElement) {