[es7] Implement async functions parsing

src/preparser.cc

Index: src/preparser.cc
diff --git a/src/preparser.cc b/src/preparser.cc
index 74e11b1d454b3b33a3f6bcadb18f91476a246f37..305070c9576dbd61f444a2fd963bf466deda2eff 100644
--- a/src/preparser.cc
+++ b/src/preparser.cc
@@ -48,6 +48,8 @@ PreParserIdentifier PreParserTraits::GetSymbol(Scanner* scanner) {
     return PreParserIdentifier::Static();
   } else if (scanner->current_token() == Token::YIELD) {
     return PreParserIdentifier::Yield();
+  } else if (scanner->current_token() == Token::ASYNC) {
+    return PreParserIdentifier::Async();
   }
   if (scanner->UnescapedLiteralMatches("eval", 4)) {
     return PreParserIdentifier::Eval();
@@ -190,6 +192,11 @@ PreParser::Statement PreParser::ParseStatementListItem(bool* ok) {
   switch (peek()) {
     case Token::FUNCTION:
       return ParseFunctionDeclaration(ok);
+    case Token::ASYNC:
+      if (IsNextAsyncFunctionKeyword()) {
+        return ParseFunctionDeclaration(ok);
+      }
+      break;
     case Token::CLASS:
       return ParseClassDeclaration(ok);
     case Token::CONST:
@@ -397,6 +404,15 @@ PreParser::Statement PreParser::ParseSubStatement(bool* ok) {
     case Token::TRY:
       return ParseTryStatement(ok);
 
+    case Token::ASYNC:
+      // Make sure async keyword is followed by a function keyword,
+      // otherwise parse 'async' as part of an expression or
+      // labelled statement (default case in this switch).
+      if (!allow_harmony_async_await() || !IsNextAsyncFunctionKeyword()) {
+        return ParseExpressionOrLabelledStatement(ok);
+      }
+
+    // Fall through.
     case Token::FUNCTION: {
       Scanner::Location start_location = scanner()->peek_location();
       Statement statement = ParseFunctionDeclaration(CHECK_OK);
@@ -436,21 +452,31 @@ PreParser::Statement PreParser::ParseSubStatement(bool* ok) {
 PreParser::Statement PreParser::ParseFunctionDeclaration(bool* ok) {
   // FunctionDeclaration ::
   //   'function' Identifier '(' FormalParameterListopt ')' '{' FunctionBody '}'
+  // AsyncFunctionDeclaration ::
+  //   'async' 'function' Identifier '(' FormalParameterListopt ')'
+  //      '{' FunctionBody '}'
   // GeneratorDeclaration ::
   //   'function' '*' Identifier '(' FormalParameterListopt ')'
   //      '{' FunctionBody '}'
+  bool is_async = allow_harmony_async_await() ? Check(Token::ASYNC) : false;
   Expect(Token::FUNCTION, CHECK_OK);
   int pos = position();
-  bool is_generator = Check(Token::MUL);
+  bool is_generator = is_async ? false : Check(Token::MUL);
   bool is_strict_reserved = false;
   Identifier name = ParseIdentifierOrStrictReservedWord(
       &is_strict_reserved, CHECK_OK);
+
+  FunctionKind kind = FunctionKind::kNormalFunction;
+  if (is_generator) {
+    kind = FunctionKind::kGeneratorFunction;
+  } else if (is_async) {
+    kind = FunctionKind::kAsyncFunction;
+  }
+
   ParseFunctionLiteral(name, scanner()->location(),
                        is_strict_reserved ? kFunctionNameIsStrictReserved
                                           : kFunctionNameValidityUnknown,
-                       is_generator ? FunctionKind::kGeneratorFunction
-                                    : FunctionKind::kNormalFunction,
-                       pos, FunctionLiteral::DECLARATION,
+                       kind, pos, FunctionLiteral::DECLARATION,
                        FunctionLiteral::NORMAL_ARITY, language_mode(),
                        CHECK_OK);
   return Statement::FunctionDeclaration();