fixes #147, add more cases to isStateless

lib/src/codegen/side_effect_analysis.dart

Index: lib/src/codegen/side_effect_analysis.dart
diff --git a/lib/src/codegen/side_effect_analysis.dart b/lib/src/codegen/side_effect_analysis.dart
index 4cccd421fccccdeddabf066261ee7d729f32a1cf..cdf8e7939a358a1f9b5c7bb2d7f11865e79376a5 100644
--- a/lib/src/codegen/side_effect_analysis.dart
+++ b/lib/src/codegen/side_effect_analysis.dart
@@ -28,6 +28,8 @@ import 'package:analyzer/src/generated/element.dart';
 /// correctness of a `true` value. However it may return `false` for things
 /// that are in fact, stateless.
 bool isStateless(Expression node, [AstNode context]) {
+  // `this` and `super` cannot be reassigned.
+  if (node is ThisExpression || node is SuperExpression) return true;
   if (node is SimpleIdentifier) {
     var e = node.staticElement;
     if (e is PropertyAccessorElement) e = e.variable;