Mark [this] as exposed if generative constructor contains function expression.

tests/language/regress_20840_test.dart

+// Copyright (c) 2014, 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 file.
+
+// Regression test for issue 20840.
+
+class SomeClass {
+  Object someField;
+
+  SomeClass() {
+    [1].forEach((o) => someMethod());
+    someField = new Object();
+  }
+
+  void someMethod() {
+    if (someField != null) {
+      throw "FAIL";
+    }
+  }
+}
+
+void main() {
+  new SomeClass();
+}
+