[es6] parse arrow ConciseBody with accept_IN flag

src/ast.h

Index: src/ast.h
diff --git a/src/ast.h b/src/ast.h
index ffae46225b525afc9718222bf4fbc82d0490bd02..ad7b277012521de7a1f6ab5ede3bfcf6118c896e 100644
--- a/src/ast.h
+++ b/src/ast.h
@@ -2547,6 +2547,11 @@ class FunctionLiteral final : public Expression {
     dont_optimize_reason_ = reason;
   }
 
+  bool accept_IN() const { return AcceptInHintBit::decode(bitfield_); }
+  void set_accept_IN(bool value) {
+    bitfield_ = AcceptInHintBit::update(bitfield_, value);
+  }
+
  protected:
   FunctionLiteral(Zone* zone, const AstRawString* name,
                   AstValueFactory* ast_value_factory, Scope* scope,
@@ -2604,6 +2609,8 @@ class FunctionLiteral final : public Expression {
   class FunctionKindBits : public BitField<FunctionKind, 6, 8> {};
   class ShouldBeUsedOnceHintBit : public BitField<ShouldBeUsedOnceHint, 15, 1> {
   };
+  // Used for lazily compiled arrow functions
+  class AcceptInHintBit : public BitField<bool, 16, 1> {};
 };