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---*\//'

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.
 

test/built-ins/Promise/S25.4.3.1_A2.3_T1.js

@@ -7,6 +7,7 @@ info: >
 es6id: S25.4.3.1_A2.3_T1
 author: Sam Mikes
 description: Promise.call(resolved Promise) throws TypeError
+flags: [async]
 ---*/
 
 var p = new Promise(function(resolve) { resolve(1); });

test/built-ins/Promise/S25.4.3.1_A2.4_T1.js

@@ -7,6 +7,7 @@ info: >
 es6id: S25.4.3.1_A2.4_T1
 author: Sam Mikes
 description: Promise.call(rejected Promise) throws TypeError
+flags: [async]
 ---*/
 
 var p = new Promise(function(resolve, reject) { reject(1) });

test/built-ins/Promise/S25.4.3.1_A4.1_T1.js

@@ -8,6 +8,7 @@ info: >
 es6id: S25.4.3.1_A4.1_T1
 author: Sam Mikes
 description: new Promise(function () { throw }) should reject
+flags: [async]
 ---*/
 
 var errorObject = {},

test/built-ins/Promise/S25.4.3.1_A5.1_T1.js

@@ -9,7 +9,7 @@ info: >
 es6id: S25.4.3.1_A5.1_T1
 author: Sam Mikes
 description: Promise executor gets default handling for 'this'
-flags: [noStrict]
+flags: [async, noStrict]
 includes: [fnGlobalObject.js]
 ---*/
 

test/built-ins/Promise/S25.4.3.1_A5.1_T2.js

@@ -9,7 +9,7 @@ info: >
 es6id: S25.4.3.1_A5.1_T2
 author: Sam Mikes
 description: Promise executor gets default handling for 'this'
-flags: [onlyStrict]
+flags: [async, onlyStrict]
 ---*/
 
 var expectedThis = undefined;

test/built-ins/Promise/all/S25.4.4.1_A2.2_T1.js

@@ -7,6 +7,7 @@ es6id: 25.4.4.1_A2.2_T1
 author: Sam Mikes
 includes: [PromiseHelper.js]
 description: Promise.all([]) returns immediately
+flags: [async]
 ---*/
 
 var sequence = [];

test/built-ins/Promise/all/S25.4.4.1_A2.3_T1.js

@@ -6,6 +6,7 @@ info: Promise.all([]) returns a promise for a new empty array
 es6id: 25.4.4.1_A2.3_T1
 author: Sam Mikes
 description: Promise.all([]) returns a promise for an array
+flags: [async]
 ---*/
 
 var arg = [];

test/built-ins/Promise/all/S25.4.4.1_A2.3_T2.js

@@ -6,6 +6,7 @@ info: Promise.all is resolved with a new empty array
 es6id: 25.4.4.1_A2.3_T2
 author: Sam Mikes
 description: Promise.all([]) returns a Promise for an empty array
+flags: [async]
 ---*/
 
 var arg = [];

test/built-ins/Promise/all/S25.4.4.1_A2.3_T3.js

@@ -6,6 +6,7 @@ info: Promise.all([]) is resolved with Promise for a new empty array
 es6id: 25.4.4.1_A2.3_T3
 author: Sam Mikes
 description: Promise.all([]) is resolved with a Promise for a new array
+flags: [async]
 ---*/
 
 var arg = [];

test/built-ins/Promise/all/S25.4.4.1_A3.1_T1.js

@@ -9,6 +9,7 @@ info: >
 es6id: 25.4.4.1_A3.1_T1
 author: Sam Mikes
 description: Promise.all(3) returns Promise rejected with TypeError
+flags: [async]
 ---*/
 
 var nonIterable = 3;

test/built-ins/Promise/all/S25.4.4.1_A3.1_T2.js

@@ -10,6 +10,7 @@ info: >
 es6id: S25.4.4.1_A3.1_T2
 author: Sam Mikes
 description: Promise.all(new Error()) returns Promise rejected with TypeError
+flags: [async]
 ---*/
 
 Promise.all(new Error("abrupt")).then(function () {

test/built-ins/Promise/all/S25.4.4.1_A3.1_T3.js

@@ -9,6 +9,7 @@ es6id: S25.4.4.1_A3.1_T3
 author: Sam Mikes
 description: Promise.all((throw on GetIterator)) returns Promise rejected with TypeError
 features: [Symbol.iterator]
+flags: [async]
 ---*/
 
 var iterThrows = {};

test/built-ins/Promise/all/S25.4.4.1_A5.1_T1.js

@@ -9,6 +9,7 @@ es6id: S25.4.4.1_A5.1_T1
 author: Sam Mikes
 description: iterator.next throws, causing Promise.all to reject
 features: [Symbol.iterator]
+flags: [async]
 ---*/
 
 var iterThrows = {};

test/built-ins/Promise/all/S25.4.4.1_A6.1_T2.js

@@ -8,6 +8,7 @@ info: >
 es6id: S25.4.4.1_A6.1_T2
 author: Sam Mikes
 description: Promise.all([]) returns a promise for an empty array
+flags: [async]
 ---*/
 
 var p = Promise.all([]);

test/built-ins/Promise/all/S25.4.4.1_A7.1_T1.js

@@ -8,6 +8,7 @@ info: >
 es6id: S25.4.4.1_A6.1_T2
 author: Sam Mikes
 description: Promise.all([p1]) is resolved with a promise for a one-element array
+flags: [async]
 ---*/
 
 var p1 = Promise.resolve(3);

test/built-ins/Promise/all/S25.4.4.1_A7.2_T1.js

@@ -9,6 +9,7 @@ es6id: S25.4.4.1_A7.2_T1
 author: Sam Mikes
 description: Promise.all() accepts a one-element array
 includes: [PromiseHelper.js]
+flags: [async]
 ---*/
 
 var sequence = [];

test/built-ins/Promise/all/S25.4.4.1_A8.1_T1.js

@@ -6,6 +6,7 @@ es6id: S25.4.4.1_A8.1_T1
 author: Sam Mikes
 description: Promise.all([p1, p2]) resolution functions are called in predictable sequence
 includes: [PromiseHelper.js]
+flags: [async]
 ---*/
 
 var sequence = [];

test/built-ins/Promise/all/S25.4.4.1_A8.2_T1.js

@@ -8,6 +8,7 @@ es6id: S25.4.4.1_A8.1_T1
 author: Sam Mikes
 description: Promise.all() rejects when a promise in its array rejects
 includes: [PromiseHelper.js]
+flags: [async]
 ---*/
 
 var rejectP1,

test/built-ins/Promise/all/S25.4.4.1_A8.2_T2.js

@@ -8,6 +8,7 @@ es6id: S25.4.4.1_A8.2_T2
 author: Sam Mikes
 description: Promise.all() rejects when second promise in array rejects
 includes: [PromiseHelper.js]
+flags: [async]
 ---*/
 
 var rejectP2,