enable lint overridden_fields (#9527)

* enable lint overridden_fields

* address review comment

.analysis_options

@@ -86,7 +86,7 @@ linter:
     # - omit_local_variable_types # opposite of always_specify_types
     # - one_member_abstracts # too many false positives
     # - only_throw_errors # https://github.com/flutter/flutter/issues/5792
-    # - overridden_fields # not yet tested
+    - overridden_fields
     - package_api_docs
     - package_prefixed_library_names
     # - parameter_assignments # we do this commonly

.analysis_options_repo

@@ -84,7 +84,7 @@ linter:
     # - omit_local_variable_types # opposite of always_specify_types
     # - one_member_abstracts # too many false positives
     # - only_throw_errors # https://github.com/flutter/flutter/issues/5792
-    # - overridden_fields # not yet tested
+    - overridden_fields
     - package_api_docs
     - package_prefixed_library_names
     # - parameter_assignments # we do this commonly

packages/flutter/lib/src/foundation/assertions.dart

@@ -105,7 +105,9 @@ class FlutterErrorDetails {
 
   /// Converts the [exception] to a string.
   ///
-  /// This applies some additional logic to 
+  /// This applies some additional logic to make [AssertionError] exceptions
+  /// prettier, to handle exceptions that stringify to empty strings, to handle
+  /// objects that don't inherit from [Exception] or [Error], and so forth.
   String exceptionAsString() {
     String longMessage;
     if (exception is AssertionError) {
@@ -151,7 +153,7 @@ class FlutterError extends AssertionError {
   /// Include as much detail as possible in the full error message,
   /// including specifics about the state of the app that might be
   /// relevant to debugging the error.
-  FlutterError(this.message);
+  FlutterError(String message) : super(message);
 
   /// The message associated with this error.
   ///
@@ -169,7 +171,7 @@ class FlutterError extends AssertionError {
   /// All sentences in the error should be correctly punctuated (i.e.,
   /// do end the error message with a period).
   @override
-  final String message;
+  String get message => super.message;
 
   @override
   String toString() => message;