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

tests/language/src/ListTest.dart

Index: tests/language/src/ListTest.dart
===================================================================
--- tests/language/src/ListTest.dart	(revision 2646)
+++ tests/language/src/ListTest.dart	(working copy)
@@ -137,6 +137,16 @@
       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);
   }
 }