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

runtime/lib/growable_array.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.
 
 class GrowableObjectArray<T> implements List<T> {
   ObjectArray<T> backingArray;
 
   factory GrowableObjectArray._uninstantiable() {
     throw const UnsupportedOperationException(
         "GrowableObjectArray can only be allocated by the VM");
@@ skipping 145 matching lines @@
     add(element);
   }
 
   void addAll(Collection<T> collection) {
     for (T elem in collection) {
       add(elem);
     }
   }
 
   T removeLast() {
+    if (_length == 0) {
+      throw new IndexOutOfRangeException(-1);
+    }
     _length--;
     return backingArray[_length];
   }
 
   T last() {
     if (_length === 0) {
       throw new IndexOutOfRangeException(-1);
     }
     return backingArray[_length - 1];
   }
@@ skipping 63 matching lines @@
     if (!hasNext()) {
       throw const NoMoreElementsException();
     }
     return _array[_pos++];
   }
 
   final GrowableObjectArray<T> _array;
   int _pos;
 }