Fix list length optimization bug: do not inline a length access to a constant if the receiver can b…

tests/language/list_length_tracer_test.dart

 // Copyright (c) 2013, 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 that a loop invariant code motion optimization does not try to
-// hoist instructions that may throw.
+// Test that dart2js' optimization on list length does not fold a
+// length getter to a constant if the receiver can be null.
 
 import "package:expect/expect.dart";
 
 var a = 42;
 var b;
 
 main() {
-  Expect.throws(() {
-    while (true) {
-      a = 54;
-      b.length;
-    }
-  });
-  b = [];
-  Expect.equals(54, a);
+  Expect.throws(() => b.length);
+  b = const [42];
 }