Make asynchronous test configuration explicit

For asynchronous tests, the contract between test file and test runner
is implicit: runners are expected to inspect the source code for
references to a global `$DONE` identifier.

Promote a more explicit contract between test file and test runner by
introducing a new frontmatter "tag", `async`. This brings asynchronous
test configuration in-line with other configuration mechanisms and also
provides a more natural means of test filtering.

The modifications to test files was made programatically using the
`grep` and `sed` utilities:

    $ grep "\$DONE" test/ -r --files-with-match --null | \
        xargs -0 sed -i 's/^\(flags:\s*\)\[/\1[async, /g'
    $ grep "\$DONE" test/ -rl --null | \
        xargs -0 grep -E '^flags:' --files-without-match --null | \
        xargs -0 sed -i 's/^---\*\//flags: [async]\n---*\//'

1 files changed, 1 insertions, 1 deletions

diff --git a/tools/packaging/test262.py b/tools/packaging/test262.py
index 1192005fc..551d6d3e0 100755
--- a/tools/packaging/test262.py
+++ b/tools/packaging/test262.py
@@ -309,7 +309,7 @@ class TestCase(object):
     return 'raw' in self.testRecord
 
   def IsAsyncTest(self):
-    return '$DONE' in self.test
+    return 'async' in self.testRecord
 
   def GetIncludeList(self):
     if self.testRecord.get('includes'):