Fix throwing when there's an assertion error

lib/index.mjs

@@ -7,11 +7,17 @@ function AssertDiff() {
 
 AssertDiff.options = {strict: false};
 
+const AsyncFunction = (async function() {}).constructor;
+
 Object.keys(assert).forEach(function(k) {
   let fun = assert[k];
-  AssertDiff[k] = async function() {
-    return fun.apply(fun, arguments);
-  };
+  AssertDiff[k] = fun.constructor instanceof AsyncFunction ?
+    async function() {
+      return fun.apply(fun, arguments);
+    } :
+    function() {
+      return fun.apply(fun, arguments);
+    };
 });
 
 export const deepStrictEqual = AssertDiff.deepStrictEqual = function deepStrictEqual() {

test/example-passing-tests.js

@@ -70,4 +70,8 @@ it('async functions', async function() {
     })
   }))
   assert.ok(c);
+
+  assert.throws(function() {
+    assert.equal(1, 2, 'should be able to throw outside of async functions too');
+  });
 })