Assert: Allow fat arrow functions in throws expected handler

Gruntfile.js

@@ -175,7 +175,8 @@ module.exports = function( grunt ) {
 				"test/module-only",
 				"test/module-skip",
 				"test/module-todo",
-				"test/es2017/async-functions"
+				"test/es2017/async-functions",
+				"test/es2017/throws"
 			]
 		},
 		"watch-repeatable": {

src/assert.js

@@ -323,7 +323,8 @@ class Assert {
 				expected = String( expected );
 
 			// Expected is a constructor, maybe an Error constructor
-			} else if ( expectedType === "function" && actual instanceof expected ) {
+			} else if ( expectedType === "function" &&
+				expected.prototype !== undefined && actual instanceof expected ) {
 				result = true;
 
 			// Expected is an Error object

test/es2017/throws.js

@@ -0,0 +1,46 @@
+
+QUnit.module( "assert" );
+
+function CustomError( message ) {
+	this.message = message;
+}
+
+CustomError.prototype.toString = function() {
+	return this.message;
+};
+
+QUnit.test( "throws", function( assert ) {
+	assert.expect( 1 );
+
+	assert.throws(
+		function() {
+			throw new CustomError( "some error description" );
+		},
+		err => {
+			return err instanceof CustomError && /description/.test( err );
+		},
+		"custom validation function"
+	);
+} );
+
+QUnit.module( "failing assertions", {
+	beforeEach: function( assert ) {
+		var originalPushResult = assert.pushResult;
+		assert.pushResult = function( resultInfo ) {
+
+			// Inverts the result so we can test failing assertions
+			resultInfo.result = !resultInfo.result;
+			originalPushResult( resultInfo );
+		};
+	}
+} );
+
+QUnit.test( "throws", function( assert ) {
+	assert.throws(
+		function() {
+			throw "foo";
+		},
+		() => false,
+		"throws fail when expected function returns false"
+	);
+} );

test/main/assert.js

@@ -542,6 +542,16 @@ QUnit.test( "throws", function( assert ) {
 		/bar/,
 		"throws fail when regexp doesn't match the error message"
 	);
+
+	assert.throws(
+		function() {
+			throw "foo";
+		},
+		function() {
+			return false;
+		},
+		"throws fail when expected function returns false"
+	);
 } );
 
 QUnit.test( "rejects", function( assert ) {