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 3be641750c792b4bd38059e725e6648be0fc240c..788f57a018e669c70b0662b0f494ca5d53eced87 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());
 }