HTML Reporter: Ensure unhandled rejection fails.

Brings unhandled rejections in line with unhandled exceptions.

reporter/html.js

@@ -979,4 +979,9 @@ export function escapeText( s ) {
 
 		return ret;
 	};
+
+	// Listen for unhandled rejections, and call QUnit.onError.
+	window.addEventListener( "unhandledrejection", function( event ) {
+		QUnit.onError( event.reason );
+	} );
 }() );

test/index.html

@@ -19,6 +19,7 @@
 	<script src="setTimeout.js"></script>
 	<script src="reporter-html/reporter-html.js"></script>
 	<script src="reporter-html/diff.js"></script>
+	<script src="reporter-html/unhandled-rejection.js"></script>
 	<script src="onerror/inside-test.js"></script>
 	<script src="onerror/outside-test.js"></script>
 </head>

test/reporter-html/unhandled-rejection.js

@@ -0,0 +1,35 @@
+// Currently (2017-12-18) only Chrome has implemented `unhandledRejection`
+const IS_CHROME = !!window.chrome;
+
+QUnit.module( "Unhandled Rejections", function( hooks ) {
+	var originalQUnitOnError;
+
+	hooks.beforeEach( function() {
+		originalQUnitOnError = QUnit.onError;
+	} );
+
+	hooks.afterEach( function() {
+		QUnit.onError = originalQUnitOnError;
+	} );
+
+	if ( IS_CHROME ) {
+		QUnit.test( "test passes just fine, but has a rejected promise", function( assert ) {
+
+			QUnit.onError = function() {
+				assert.ok( true, "QUnit.onError was called" );
+			};
+
+			const done = assert.async();
+
+			new Promise( function( resolve ) {
+				setTimeout( resolve );
+			} )
+				.then( function() {
+					throw new Error( "Error thrown in non-returned promise!" );
+				} );
+
+			// prevent test from exiting before unhandled rejection fires
+			setTimeout( done, 10 );
+		} );
+	}
+} );