A return and a throw should empty the environment.

tests/language/src/ReturnInLoopTest.dart

Index: tests/language/src/ReturnInLoopTest.dart
===================================================================
--- tests/language/src/ReturnInLoopTest.dart	(revision 0)
+++ tests/language/src/ReturnInLoopTest.dart	(revision 0)
@@ -0,0 +1,23 @@
+// Copyright (c) 2012, 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.
+// Test for a leg bug where the live environment was not computed
+// right.
+
+class A {
+  foo() {
+    var x = 0;
+    while (true) {
+      if (true) {
+        return 42;
+      } else {
+      }
+      x = bar();
+    }
+  }
+  bar() => 1;
+}
+
+main() {
+  Expect.equals(42, new A().foo());
+}