[esnext] prototype runtime implementation for async functions

src/builtins.h

Index: src/builtins.h
diff --git a/src/builtins.h b/src/builtins.h
index 44143514111bc4b2bc12e552dad19293abc7f244..c285541d3d9d3fe55bc5db88b6d9c2dd76294dcb 100644
--- a/src/builtins.h
+++ b/src/builtins.h
@@ -312,6 +312,8 @@ inline bool operator&(BuiltinExtraArguments lhs, BuiltinExtraArguments rhs) {
   V(GeneratorPrototypeNext, 2)    \
   V(GeneratorPrototypeReturn, 2)  \
   V(GeneratorPrototypeThrow, 2)   \
+  V(AsyncFunctionNext, 2)         \
+  V(AsyncFunctionThrow, 2)        \
   V(MathCeil, 2)                  \
   V(MathClz32, 2)                 \
   V(MathFloor, 2)                 \
@@ -635,6 +637,9 @@ class Builtins {
   // ES6 section 25.3.1.4 Generator.prototype.throw ( exception )
   static void Generate_GeneratorPrototypeThrow(CodeStubAssembler* assembler);
 
+  static void Generate_AsyncFunctionNext(CodeStubAssembler* assembler);
+  static void Generate_AsyncFunctionThrow(CodeStubAssembler* assembler);
+
   // ES6 section 19.1.3.2 Object.prototype.hasOwnProperty
   static void Generate_ObjectHasOwnProperty(CodeStubAssembler* assembler);