Add a test that verifies that rethrow ("throw;") throws the expected exception.

tests/language/src/RethrowTest.dart

Index: tests/language/src/RethrowTest.dart
===================================================================
--- tests/language/src/RethrowTest.dart	(revision 0)
+++ tests/language/src/RethrowTest.dart	(revision 0)
@@ -0,0 +1,61 @@
+// Copyright (c) 2011, 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.
+// Dart test program for testing throw statement
+
+class MyException {
+  const MyException();
+}
+
+class OtherException {
+  const OtherException();
+}
+
+class RethrowTest {
+  MyException currentException;
+
+  void throwException() {
+    currentException = new MyException();
+    throw currentException;
+  }
+
+  void testRethrowPastUncaught() {
+    try {
+      try {
+        try {
+          throwException();
+          Expect.fail("Should have thrown an exception");
+        } catch (var e) {
+          Expect.equals(true, e === currentException);
+          throw;
+          Expect.fail("Should have thrown an exception");
+        }
+      } catch (OtherException e) {
+        Expect.fail("Should not have caught OtherException");
+      }
+    } catch (var e) {
+      Expect.equals(true, e === currentException);
+    }
+  }
+
+  void testRethrow() {
+    try {
+      try {
+        throwException();
+        Expect.fail("Should have thrown an exception");
+      } catch (var e) {
+        Expect.equals(true, e === currentException);
+        throw;
+        Expect.fail("Should have thrown an exception");
+      }
+    } catch (var e) {
+      Expect.equals(true, e === currentException);
+    }
+  }
+}
+
+main() {
+  RethrowTest t = new RethrowTest();
+  t.testRethrow();
+  t.testRethrowPastUncaught();
+}