Fix issue 920: do not allow length of a list go below 0.

tests/language/src/ListTest.dart

 // 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 arrays.
 
 class ListTest {
   static void TestIterator() {
     List<int> a = new List<int>(10);
     int count = 0;
 
@@ skipping 119 matching lines @@
     }
     Expect.equals(true, exceptionCaught);
 
     exceptionCaught = false;
     try {
       var a = new List(99999999999999999999999);  // Non-Smi.
     } catch (Exception e) {  // Must agree which exception to throw.
       exceptionCaught = true;
     }
     Expect.equals(true, exceptionCaught);
+
+    exceptionCaught = false;
+    List list = new List();
+    try {
+      list.removeLast();
+    } catch (IndexOutOfRangeException e) {
+      exceptionCaught = true;
+    }
+    Expect.equals(0, list.length);
+    Expect.equals(true, exceptionCaught);
   }
 }
 
 main() {
   ListTest.testMain();
 }