[es6] parse arrow ConciseBody with accept_IN flag

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 100da7d766e86c8f57b7712b6216d0b72fc39dfa..788cac9c135ce6f4e53e087ba679e45dfe7201c2 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -6710,6 +6710,10 @@ class SharedFunctionInfo: public HeapObject {
   // Indicates that the the shared function info has never been compiled before.
   DECL_BOOLEAN_ACCESSORS(never_compiled)
 
+  // Indicates (for non-eager compilation) that a function's body can be parsed
+  // accepting [In] grammar production variants.
+  DECL_BOOLEAN_ACCESSORS(accept_IN)
+
   inline FunctionKind kind();
   inline void set_kind(FunctionKind kind);
 
@@ -6983,6 +6987,7 @@ class SharedFunctionInfo: public HeapObject {
     kIsAsmFunction,
     kDeserialized,
     kNeverCompiled,
+    kAcceptIn,
     kCompilerHintsCount  // Pseudo entry
   };
   // Add hints for other modes when they're added.