Allow rethrow in nested try statements

tests/language/regress_26530_test.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 file.
+
+import "package:expect/expect.dart";
+
+var trace = "";
+
+main() {
+  var x = 0;
+  try {
+    try {
+      throw x++;  // 1
+    } on int catch (e) {
+      trace += "$e";
+      trace += "-$x";
+      x++;  // 2
+      try {
+        x++;  // 3
+        rethrow;
+      } finally {
+        trace += "-f";
+        x++;  // 4
+      }
+    }
+  } catch (e) {
+    trace += "-c";
+    trace += "-$e";
+    trace += "-$x";
+  }
+  Expect.equals("0-1-f-c-0-4", trace);
+}