VM: Separate precompilation-specific code, make flags const.

runtime/vm/flags.h

Index: runtime/vm/flags.h
diff --git a/runtime/vm/flags.h b/runtime/vm/flags.h
index d3e4869f28f6a9e858eb9a56c265098590f84a73..7e265d25ce64613e24dada1040e37af9cfc51544 100644
--- a/runtime/vm/flags.h
+++ b/runtime/vm/flags.h
@@ -121,12 +121,26 @@ class Flags {
 
 #endif  // defined(PRODUCT)
 
+#if defined(DART_PRECOMPILED_RUNTIME)

[Ivan Posva, 2016/02/19 01:07:49]

As discussed we need to have a way for a flag to be unchangeable even in
non-precompiled product.

[Florian Schneider, 2016/02/19 17:42:29]

Done.

+#define PRECOMPILE_FLAG_MARCO(name, pre_value, type, default_value, comment)   \
+  const type FLAG_##name = pre_value;
+
+#else  // defined(DART_PRECOMPILED_RUNTIME)
+#define PRECOMPILE_FLAG_MARCO(name, pre_value, type, default_value, comment)   \
+  extern type FLAG_##name;
+
+#endif  // defined(DART_PRECOMPILED_RUNTIME)
+
 // Now declare all flags here.
-FLAG_LIST(PRODUCT_FLAG_MARCO, RELEASE_FLAG_MARCO, DEBUG_FLAG_MARCO)
+FLAG_LIST(PRODUCT_FLAG_MARCO,
+          RELEASE_FLAG_MARCO,
+          DEBUG_FLAG_MARCO,
+          PRECOMPILE_FLAG_MARCO)
 
 #undef RELEASE_FLAG_MARCO
 #undef DEBUG_FLAG_MARCO
 #undef PRODUCT_FLAG_MARCO
+#undef PRECOMPILE_FLAG_MARCO
 
 }  // namespace dart