Check for no constructors before iterating (issue 28703)

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 4718171ab6e33ea0d2029acca8287c711599739f..2e0d063e80d1220a2c72c794010625cd64a073d0 100644
--- a/pkg/analyzer/lib/src/dart/constant/evaluation.dart
+++ b/pkg/analyzer/lib/src/dart/constant/evaluation.dart
@@ -305,12 +305,14 @@ class ConstantEvaluationEngine {
         bool defaultSuperInvocationNeeded = true;
         List<ConstructorInitializer> initializers =
             constant.constantInitializers;
-        for (ConstructorInitializer initializer in initializers) {
-          if (initializer is SuperConstructorInvocation ||
-              initializer is RedirectingConstructorInvocation) {
-            defaultSuperInvocationNeeded = false;
+        if (initializers != null) {
+          for (ConstructorInitializer initializer in initializers) {
+            if (initializer is SuperConstructorInvocation ||
+                initializer is RedirectingConstructorInvocation) {
+              defaultSuperInvocationNeeded = false;
+            }
+            initializer.accept(referenceFinder);
           }
-          initializer.accept(referenceFinder);
         }
         if (defaultSuperInvocationNeeded) {
           // No explicit superconstructor invocation found, so we need to