In background compilation make a copy of Field in order to freeze its state

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
diff --git a/runtime/vm/intermediate_language.cc b/runtime/vm/intermediate_language.cc
index 0a67661e7074a439e668b91ca3d8725e97405c9b..f784c2a244834342e79a5b6f37e84952b935c001 100644
--- a/runtime/vm/intermediate_language.cc
+++ b/runtime/vm/intermediate_language.cc
@@ -419,6 +419,7 @@ ConstantInstr::ConstantInstr(const Object& value, TokenPosition token_pos)
   ASSERT(!value.IsBigint() || !Bigint::Cast(value).FitsIntoSmi());
   ASSERT(!value.IsBigint() || !Bigint::Cast(value).FitsIntoInt64());
   ASSERT(!value.IsMint() || !Smi::IsValid(Mint::Cast(value).AsInt64Value()));
+  ASSERT(!value.IsField() || Field::Cast(value).IsOriginal());
 }