Add support for configurable -OX optimization levels for debug builds

runtime/BUILD.gn

Index: runtime/BUILD.gn
diff --git a/runtime/BUILD.gn b/runtime/BUILD.gn
index 36983b6a75caede18bcd48379045163686f254f8..1357ea29c5e96766813166455c323ebbab013dc6 100644
--- a/runtime/BUILD.gn
+++ b/runtime/BUILD.gn
@@ -28,6 +28,9 @@ declare_args() {
   dart_target_arch = ""
 
   dart_experimental_interpreter = false
+
+  # The optimization level to use for debug builds.
+  dart_debug_optimization_level = 2
 }
 
 config("dart_public_config") {
@@ -163,7 +166,7 @@ config("dart_config") {
     ]
 
     if (dart_debug) {
-      cflags += [ "-O2" ]
+      cflags += [ "-O${dart_debug_optimization_level}" ]

[Kevin Millikin (Google), 2017/01/23 13:32:22]

This flag seems to be injected two places: once here and once in
build/config/compiler/BUILD.gn.

Is it the lower one that takes precedence?  The second one?

[kustermann, 2017/01/23 13:59:11]

Yes, afaik.

[kustermann, 2017/01/23 14:11:54]

Sorry, to clarify: It is the second one.

"""
If you use multiple -O options, with or without level numbers, the last such
option is the one that is effective.
"""

[zra, 2017/01/23 16:39:54]

We probably need to do this in both places, both for clarity and also to make
this work on Windows. This setting also does not cover third_party dependencies.

I think we should do this by adding a GN argument to
build/config/compiler/BUILD.gn, like "debug_optimization_level" in addition to
this one. (We need both so that we can override this one, and to set the debug
optimization level from the Flutter engine build.) In our gn.py, we'd set them
both to the same thing.

[kustermann, 2017/01/24 11:44:51]

Done.
As a VM developer I would like to not have "<optimized out>" issues with gdb in
VM code. I would be perfectly fine with having tcmalloc/boringssl/embedder/...
compile with -O2 so the tests run faster. But I don't really have a strong
opinion on this.

     } else {
       cflags += [ "-O3" ]
     }