Be less aggressive in sharing contexts between scopes so that sibling contexts

tests/language/regress_22858_test.dart

+// Copyright (c) 2015, 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.
+
+import 'package:expect/expect.dart';
+
+main() {
+  var good = "good";
+
+  f1() {
+    {
+      var bad = "bad";
+      f2() { bad; }
+    }
+
+    Expect.equals("good", good);
+    do {
+      Expect.equals("good", good);
+      int ugly = 0;
+      f3() { ugly; }
+    } while (false);
+  }
+
+  f1();
+}