Change GN debug builds to use -O2, which is the value used by GYP.

runtime/BUILD.gn

 # Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 declare_args() {
   # Instead of using is_debug, we introduce a different flag for specifying a
   # Debug build of Dart so that clients can still use a Release build of Dart
   # while themselves doing a Debug build.
   dart_debug = false
 
@@ skipping 145 matching lines @@
       "-Wvla",
       "-Wno-conversion-null",
       "-Woverloaded-virtual",
       "-g3",
       "-ggdb3",
       "-fno-rtti",
       "-fno-exceptions",
     ]
 
     if (dart_debug) {
-      cflags += [ "-O1" ]
+      cflags += [ "-O2" ]
     } else {
       cflags += [ "-O3" ]
     }
 
     ldflags = []
     if (defined(is_asan) && is_asan) {
       ldflags += [ "-fsanitize=address" ]
     }
     if (defined(is_msan) && is_msan) {
       ldflags += [ "-fsanitize=memory" ]
@@ skipping 124 matching lines @@
     "third_party/double-conversion/src:libdouble_conversion",
     "vm:libdart_lib",
     "vm:libdart_lib_nosnapshot",
     "vm:libdart_vm",
     "vm:libdart_vm_nosnapshot",
   ]
   sources = [
     "vm/libdart_dependency_helper.cc",
   ]
 }