fix #29233, final fields can be settable in a mock

pkg/dev_compiler/lib/src/compiler/element_helpers.dart

Index: pkg/dev_compiler/lib/src/compiler/element_helpers.dart
diff --git a/pkg/dev_compiler/lib/src/compiler/element_helpers.dart b/pkg/dev_compiler/lib/src/compiler/element_helpers.dart
index 79b3feb173e4626936d1840aea8a640a4769f020..0937565ae7f1e3691c226841728ddc61351abde6 100644
--- a/pkg/dev_compiler/lib/src/compiler/element_helpers.dart
+++ b/pkg/dev_compiler/lib/src/compiler/element_helpers.dart
@@ -155,5 +155,21 @@ List<ClassElement> getImmediateSuperclasses(ClassElement c) {
   return result;
 }
 
+/// Returns true if the library [l] is dart:_runtime.
+// TODO(jmesserly): unlike other methods in this file, this one wouldn't be
+// suitable for upstream to Analyzer, as it's DDC specific.
 bool isSdkInternalRuntime(LibraryElement l) =>
     l.isInSdk && l.source.uri.toString() == 'dart:_runtime';
+
+/// Return `true` if the given [classElement] has a noSuchMethod() method
+/// distinct from the one declared in class Object, as per the Dart Language
+/// Specification (section 10.4).
+// TODO(jmesserly): this was taken from error_verifier.dart
+bool hasNoSuchMethod(ClassElement classElement) {
+  // TODO(jmesserly): this is slow in Analyzer. It's a linear scan through all
+  // methods, up through the class hierarchy.
+  var method = classElement.lookUpMethod(
+      FunctionElement.NO_SUCH_METHOD_METHOD_NAME, classElement.library);
+  var definingClass = method?.enclosingElement;
+  return definingClass != null && !definingClass.type.isObject;
+}