Fix product build on Linux clang.

runtime/vm/code_generator.cc

Index: runtime/vm/code_generator.cc
diff --git a/runtime/vm/code_generator.cc b/runtime/vm/code_generator.cc
index 54c48c881e0097382e05bbeeeedd1c35edd482ad..24c8c8cd26d05ba62af8848128b1b317f629c828 100644
--- a/runtime/vm/code_generator.cc
+++ b/runtime/vm/code_generator.cc
@@ -707,6 +707,7 @@ static void CheckResultError(const Object& result) {
 DEFINE_RUNTIME_ENTRY(BreakpointRuntimeHandler, 0) {
   if (!FLAG_support_debugger) {
     UNREACHABLE();
+    return;
   }
   DartFrameIterator iterator;
   StackFrame* caller_frame = iterator.NextFrame();
@@ -725,6 +726,7 @@ DEFINE_RUNTIME_ENTRY(BreakpointRuntimeHandler, 0) {
 DEFINE_RUNTIME_ENTRY(SingleStepHandler, 0) {
   if (!FLAG_support_debugger) {
     UNREACHABLE();
+    return;
   }
   const Error& error =
       Error::Handle(isolate->debugger()->DebuggerStepCallback());