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..5c5c9c9bea3ec8b0468975dd34249a576b4672d7 100644
--- a/runtime/vm/code_generator.cc
+++ b/runtime/vm/code_generator.cc
@@ -708,6 +708,7 @@ DEFINE_RUNTIME_ENTRY(BreakpointRuntimeHandler, 0) {
   if (!FLAG_support_debugger) {
     UNREACHABLE();

[Cutch, 2016/02/08 19:02:55]

I'm pretty sure all that is needed is a return statement after the
UNREACHABLE().

[rmacnak, 2016/02/08 19:05:37]

Yes, much better.

   }
+#if !defined(PRODUCT)
   DartFrameIterator iterator;
   StackFrame* caller_frame = iterator.NextFrame();
   ASSERT(caller_frame != NULL);
@@ -719,6 +720,7 @@ DEFINE_RUNTIME_ENTRY(BreakpointRuntimeHandler, 0) {
     UNREACHABLE();
   }
   arguments.SetReturn(orig_stub);
+#endif  // !PRODUCT
 }
 
 
@@ -726,12 +728,14 @@ DEFINE_RUNTIME_ENTRY(SingleStepHandler, 0) {
   if (!FLAG_support_debugger) {
     UNREACHABLE();
   }
+#if !defined(PRODUCT)
   const Error& error =
       Error::Handle(isolate->debugger()->DebuggerStepCallback());
   if (!error.IsNull()) {
     Exceptions::PropagateError(error);
     UNREACHABLE();
   }
+#endif  // !PRODUCT
 }