Migrate fixnum tests to unittest. Fix int32 rollover bug.

pkg/pkg.status

 # Copyright (c) 2012, 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.
 
-# Run this test manually to verify that the fixnum library produces
-# the same results as native ints on a set of directed and random inputs.
-# Skip it when running automated tests because it times out.  This
-# test only makes sense on runtimes that support 64-bit integer
-# arithmetic natively, i.e., the VM.
-fixnum/test/int_64_vm_test: Skip
-
 # Don't run any test-like files that show up in packages directories. It
 # shouldn't be necessary to run "pub install" in these packages, but if you do
 # it shouldn't break the tests.
 */packages/*/*: Skip
 */*/packages/*/*: Skip
 */*/*/packages/*/*: Skip
 
 # Skip non-test files ending with "_test".
 scheduled_test/lib/*: Skip
 
@@ skipping 164 matching lines @@
 [ ($runtime == dartium || $runtime == drt) && $compiler == none ]
 path/test/dart2js_test: Fail, OK # Issue 6490
 
 # Skip mdv tests on command line VM, they only run in the browser.
 [ $runtime == vm ]
 mdv: Skip
 
 [ $runtime == safari || $runtime == chrome || $runtime == ie9 || $runtime == ff || $runtime == dartium || $runtime == drt ]
 docgen/test/single_library_test: Skip # Uses dart:io