Print messages when aborting background compilation (--trace-compiler); turn on background compilat…

runtime/vm/intermediate_language_arm.cc

Index: runtime/vm/intermediate_language_arm.cc
diff --git a/runtime/vm/intermediate_language_arm.cc b/runtime/vm/intermediate_language_arm.cc
index dfe3fc506f09f456a687c3c39332d22f342c22e0..c7591b5edb5d7eaa63a122f01b888350a7d64c78 100644
--- a/runtime/vm/intermediate_language_arm.cc
+++ b/runtime/vm/intermediate_language_arm.cc
@@ -1581,7 +1581,8 @@ void GuardFieldClassInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   if (field_cid == kDynamicCid) {
     if (Compiler::IsBackgroundCompilation()) {
       // Field state changed while compiling.
-      Compiler::AbortBackgroundCompilation(deopt_id());
+      Compiler::AbortBackgroundCompilation(deopt_id(),
+          "GuardFieldClassInstr: field state changed while compiling");
     }
     ASSERT(!compiler->is_optimizing());
     return;  // Nothing to emit.
@@ -1739,7 +1740,8 @@ void GuardFieldLengthInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   if (field().guarded_list_length() == Field::kNoFixedLength) {
     if (Compiler::IsBackgroundCompilation()) {
       // Field state changed while compiling.
-      Compiler::AbortBackgroundCompilation(deopt_id());
+      Compiler::AbortBackgroundCompilation(deopt_id(),
+          "GuardFieldLengthInstr: field state changed while compiling");
     }
     ASSERT(!compiler->is_optimizing());
     return;  // Nothing to emit.