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, 3 insertions, 1 deletions

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index da3971ea5..127673563 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -128,6 +128,8 @@ This tag is for boolean properties associated with the test.
 - **`raw`** - execute the test without any modification (no helpers will be
   available); necessary to test the behavior of directive prologue; implies
   `noStrict`
+- **`async`** - defer interpretation of test results until after the invocation
+  of the global $DONE` function
 
 #### features
 **features**: [list]
@@ -198,7 +200,7 @@ assert.throws(ReferenceError, function() {
 
 ## Writing Asynchronous Tests
 
-An asynchronous test is any test that includes the string `$DONE` anywhere in the test file.  The test runner checks for the presence of this string; if it is found, the runner expects that the `$DONE()` function will be called to signal test completion.
+An asynchronous test is any test that include the `async` frontmatter flag. When executing such tests, the runner expects that the global `$DONE()` function will be called to signal test completion.
 
  * If the argument to `$DONE` is omitted, is `undefined`, or is any other falsy value, the test is considered to have passed.