Avoid crashing on synthetic elements.

test/kernel/regression/unresolved_recovery.dart

+// Copyright (c) 2016, the Dart project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE.md file.
+
+class E {
+  foo() {
+    super[4] = 42;
+    super[4] += 5;
+    return super[2];
+  }
+}
+
+beforeTestMissingTry () {
+  // Referring to this function before it has been resolved would lead to a
+  // crash.
+  testMissingTry();
+}
+
+testMissingTry() {
+  on Exception catch (e) { }
+}
+
+main() {
+}