Change default dummy value for String to contain some info

PiperOrigin-RevId: 568789232

lib/src/builder.dart

@@ -1567,8 +1567,6 @@ class _MockClassInfo {
           ..url = 'dart:async'
           ..types.add(elementType);
       }).property('empty').call([]);
-    } else if (type.isDartCoreString) {
-      return literalString('');
     } else if (type.isDartTypedDataSealed) {
       // These types (XXXList + ByteData) from dart:typed_data are
       // sealed, e.g. "non-subtypeable", but they

lib/src/dummies.dart

@@ -16,15 +16,19 @@ import 'dart:async';
 import 'dart:collection';
 import 'dart:typed_data';
 
-import 'mock.dart' show FakeFunctionUsedError;
+import 'mock.dart' show FakeFunctionUsedError, PrettyString;
 import 'platform_dummies_js.dart'
     if (dart.library.io) 'platform_dummies_vm.dart';
 
 // TODO(yanok): try to change these to _unreasonable_ values, for example,
 // String could totally contain an explanation.
 const int _dummyInt = 0;
 const double _dummyDouble = 0.0;
-const String _dummyString = '';
+
+// Create a dummy String with info on why it was created.
+String _dummyString(Object o, Invocation i) =>
+    '''Dummy String created while calling ${i.toPrettyString()} on $o.
+''';
 
 // This covers functions with up to 20 positional arguments, for more arguments,
 // type arguments or named arguments, users would have to provide a dummy
@@ -104,7 +108,7 @@ Map<Type, DummyBuilder> _defaultDummyBuilders = {
   int: (_, _i) => _dummyInt,
   num: (_, _i) => _dummyInt,
   double: (_, _i) => _dummyDouble,
-  String: (_, _i) => _dummyString,
+  String: _dummyString,
   Int8List: (_, _i) => Int8List(0),
   Int16List: (_, _i) => Int16List(0),
   Int32List: (_, _i) => Int32List(0),

lib/src/mock.dart

@@ -1270,7 +1270,7 @@ void resetMockitoState() {
   resetDummyBuilders();
 }
 
-extension on Invocation {
+extension PrettyString on Invocation {
   /// Returns a pretty String representing a method (or getter or setter) call
   /// including its arguments, separating elements with newlines when it should
   /// improve readability.