Ignore "must_be_immutable" warning in generated files.

Mocks cannot be made immutable anyway, but this way users aren't prevented from using generated mocks altogether.

PiperOrigin-RevId: 597233416

Author: oprypin

CHANGELOG.md

@@ -1,3 +1,9 @@
+## 5.4.5-wip
+
+* Ignore "must_be_immutable" warning in generated files. Mocks cannot be made
+  immutable anyway, but this way users aren't prevented from using generated
+  mocks altogether.
+
 ## 5.4.4
 
 * Use `posix` style for local imports.

lib/src/builder.dart

@@ -123,6 +123,7 @@ class MockBuilder implements Builder {
       // example.
       b.body.add(Code(
           '// ignore_for_file: invalid_use_of_visible_for_testing_member\n'));
+      b.body.add(Code('// ignore_for_file: must_be_immutable\n'));
       b.body.add(Code('// ignore_for_file: prefer_const_constructors\n'));
       // The code_builder `asA` API unconditionally adds defensive parentheses.
       b.body.add(Code('// ignore_for_file: unnecessary_parenthesis\n'));