Implement a promise_rejects function to match assert_throws.

docs/api.md

@@ -129,21 +129,22 @@ a resolve reaction that verifies the returned value.
 Note that in the promise chain constructed in `test_function` assertions don't
 need to wrapped in `step` or `step_func` calls.
 
-Here's another example where the `test_function` uses the provided `test`
-parameter to test a Promise that is expected to reject. Note that it's important
-to handle all expected rejections since an unhandled rejection causes the test
-to fail.
+`promise_rejects` can be used to test Promises that need to reject:
+
+    promise_rejects(test_object, code, promise)
+
+The `code` argument is equivalent to the same argument to the `assert_throws`
+function.
+
+Here's an example where the `bar()` function returns a Promise that rejects
+with a TypeError:
 
     function bar() {
-      return Promise.reject("bar");
+      return Promise.reject(new TypeError());
     }
 
     promise_test(function(t) {
-      return bar()
-        .then(t.unreached_func("bar() should not accept"),
-              function(result) {
-                assert_equals(result, "bar", "bar should return 'bar'");
-              });
+      return promise_rejects(t, new TypeError(), bar);
     }, "Another example");
 
 ## Single Page Tests ##

testharness.js

@@ -464,6 +464,12 @@ policies and contribution forms [3].
                 }));
     }
 
+    function promise_rejects(test, expected, promise) {
+        return promise.then(test.unreached_func("Should have rejected.")).catch(function(e) {
+            assert_throws(expected, function() { throw e });
+        });
+    }
+
     function setup(func_or_properties, maybe_properties)
     {
         var func = null;
@@ -511,6 +517,7 @@ policies and contribution forms [3].
     expose(test, 'test');
     expose(async_test, 'async_test');
     expose(promise_test, 'promise_test');
+    expose(promise_rejects, 'promise_rejects');
     expose(generate_tests, 'generate_tests');
     expose(setup, 'setup');
     expose(done, 'done');