Check for no constructors before iterating (issue 28703)

pkg/analyzer/lib/src/dart/element/element.dart

Index: pkg/analyzer/lib/src/dart/element/element.dart
diff --git a/pkg/analyzer/lib/src/dart/element/element.dart b/pkg/analyzer/lib/src/dart/element/element.dart
index ecd8ec6ac82916ea699315c9d50859eab7b2b451..ef4007bc742e042b75f2856461a55dcce8ab5390 100644
--- a/pkg/analyzer/lib/src/dart/element/element.dart
+++ b/pkg/analyzer/lib/src/dart/element/element.dart
@@ -2006,6 +2006,10 @@ class ConstructorElementImpl extends ExecutableElementImpl
       UnlinkedExecutable serializedExecutable, ClassElementImpl enclosingClass)
       : super.forSerialized(serializedExecutable, enclosingClass);
 
+  /**
+   * Return the constant initializers for this element, which will be empty if
+   * there are no initializers, or `null` if there was an error in the source.
+   */
   List<ConstructorInitializer> get constantInitializers {
     if (serializedExecutable != null && _constantInitializers == null) {
       _constantInitializers ??= serializedExecutable.constantInitializers