[async-iteration] improve Function.prototype.toString() output

Currently, async generators are stringified the same way normal
Generators are. This change prefixes async generator methods with
"async *", and other async generator functions with
"async function* ".

BUG=v8:5855
R=adamk@chromium.org, littledan@chromium.org, jwolfe@igalia.com

Change-Id: Ia809fad64caac4464dbc9f7fa7728584d0f67832
Reviewed-on: https://chromium-review.googlesource.com/463526
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44278}

src/objects.cc

@@ -13052,13 +13052,17 @@ Handle<String> JSFunction::ToString(Handle<JSFunction> function) {
   FunctionKind kind = shared_info->kind();
   if (!IsArrowFunction(kind)) {
     if (IsConciseMethod(kind)) {
-      if (IsGeneratorFunction(kind)) {
+      if (IsAsyncGeneratorFunction(kind)) {
+        builder.AppendCString("async *");
+      } else if (IsGeneratorFunction(kind)) {
         builder.AppendCharacter('*');
       } else if (IsAsyncFunction(kind)) {
         builder.AppendCString("async ");
       }
     } else {
-      if (IsGeneratorFunction(kind)) {
+      if (IsAsyncGeneratorFunction(kind)) {
+        builder.AppendCString("async function* ");
+      } else if (IsGeneratorFunction(kind)) {
         builder.AppendCString("function* ");
       } else if (IsAsyncFunction(kind)) {
         builder.AppendCString("async function ");

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

@@ -147,6 +147,28 @@ assertEquals(AsyncGeneratorFunction.prototype.prototype,
              Object.getPrototypeOf(new AsyncGeneratorFunction().prototype));
 assertTrue(new AsyncGeneratorFunction().hasOwnProperty("prototype"));
 
+// ----------------------------------------------------------------------------
+// Basic Function.prototype.toString()
+async function* asyncGeneratorForToString() {}
+assertEquals("async function* asyncGeneratorForToString() {}",
+             asyncGeneratorForToString.toString());
+
+assertEquals("async function* () {}", async function*() {}.toString());
+assertEquals("async function* namedAsyncGeneratorForToString() {}",
+             async function* namedAsyncGeneratorForToString() {}.toString());
+
+assertEquals("async *method() { }",
+             ({ async *method() { } }).method.toString());
+assertEquals("async *method() { }",
+             (class { static async *method() { } }).method.toString());
+assertEquals("async *method() { }",
+             (new (class { async *method() { } })).method.toString());
+
+assertEquals("async function* anonymous() {\n\n}",
+             AsyncGeneratorFunction().toString());
+assertEquals("async function* anonymous() {\n\n}",
+             (new AsyncGeneratorFunction()).toString());
+
 // ----------------------------------------------------------------------------
 // AsyncGenerator functions syntactically allow AwaitExpressions
 assertEquals(1, async function*(a) { await 1; }.length);