Avoid crashing on synthetic elements.

test/kernel/regression/unresolved_recovery.dart.txt

+library;
+import self as self;
+import "dart:core" as core;
+
+class E extends core::Object {
+  constructor •() → self::E
+    : super core::Object::•();
+  method foo() → dynamic {
+    let final dynamic #t1 = 4 in throw core::dart.core::_unresolvedSuperMethodError(this, #[]=, <dynamic>[#t1, 42], <dynamic, dynamic>{}, null);
+    let final dynamic #t2 = 4 in throw core::dart.core::_unresolvedSuperMethodError(this, #[]=, <dynamic>[#t2, (throw core::dart.core::_unresolvedSuperMethodError(this, #[], <dynamic>[#t2], <dynamic, dynamic>{}, null)).+(5)], <dynamic, dynamic>{}, null);
+    return let final dynamic #t3 = 2 in throw core::dart.core::_unresolvedSuperMethodError(this, #[], <dynamic>[#t3], <dynamic, dynamic>{}, null);
+  }
+}
+static method beforeTestMissingTry() → dynamic {
+  throw core::dart.core::_genericNoSuchMethod(null, #testMissingTry, <dynamic>[], <dynamic, dynamic>{}, null);
+}
+static method testMissingTry() → dynamic
+  invalid-statement;
+static method main() → dynamic {}