Bailout if field state changed during background compilation.

runtime/vm/intermediate_language_x64.cc

Index: runtime/vm/intermediate_language_x64.cc
diff --git a/runtime/vm/intermediate_language_x64.cc b/runtime/vm/intermediate_language_x64.cc
index 53ca97b1a7d78144506d13b895ea527819037e27..1f840fd9211b9782ca373355fee1977cb67668cf 100644
--- a/runtime/vm/intermediate_language_x64.cc
+++ b/runtime/vm/intermediate_language_x64.cc
@@ -7,6 +7,7 @@
 
 #include "vm/intermediate_language.h"
 
+#include "vm/compiler.h"
 #include "vm/dart_entry.h"
 #include "vm/flow_graph.h"
 #include "vm/flow_graph_compiler.h"
@@ -1457,6 +1458,10 @@ void GuardFieldClassInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   const intptr_t nullability = field().is_nullable() ? kNullCid : kIllegalCid;
 
   if (field_cid == kDynamicCid) {
+    if (Compiler::IsBackgroundCompilation()) {
+      // Field state changed while compiling.
+      Compiler::AbortBackgroundCompilation(deopt_id());
+    }
     ASSERT(!compiler->is_optimizing());
     return;  // Nothing to emit.
   }
@@ -1598,6 +1603,10 @@ LocationSummary* GuardFieldLengthInstr::MakeLocationSummary(Zone* zone,
 
 void GuardFieldLengthInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   if (field().guarded_list_length() == Field::kNoFixedLength) {
+    if (Compiler::IsBackgroundCompilation()) {
+      // Field state changed while compiling.
+      Compiler::AbortBackgroundCompilation(deopt_id());
+    }
     ASSERT(!compiler->is_optimizing());
     return;  // Nothing to emit.
   }