[interpreter] Don't assume generator functions do an initial yield.

test/mjsunit/harmony/async-await-basic.js

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Flags: --harmony-async-await --allow-natives-syntax
 
 // Do not install `AsyncFunction` constructor on global object
 
 function assertThrowsAsync(run, errorType, message) {
   var actual;
@@ skipping 351 matching lines @@
 
 // Regress v8:5148
 assertEqualsAsync("1", () => (async({ a = NaN }) => a)({ a: "1" }));
 assertEqualsAsync(
     "10", () => (async(foo, { a = NaN }) => foo + a)("1", { a: "0" }));
 assertEqualsAsync("2", () => (async({ a = "2" }) => a)({ a: undefined }));
 assertEqualsAsync(
     "20", () => (async(foo, { a = "0" }) => foo + a)("2", { a: undefined }));
 assertThrows(() => eval("async({ foo = 1 })"), SyntaxError);
 assertThrows(() => eval("async(a, { foo = 1 })"), SyntaxError);
+
+// https://bugs.chromium.org/p/chromium/issues/detail?id=628984
+async function gaga() {
+  let i = 1;
+  while (i-- > 0) { await 42 }
+}
+assertDoesNotThrow(gaga);