Make the debug optimization level configurable through a variable.

tools/gyp/configurations_make.gypi

 # Copyright (c) 2011, 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.
 
 {
   'variables': {
     'arm_cross_libc%': '/opt/codesourcery/arm-2009q1/arm-none-linux-gnueabi/libc',
+    'dart_debug_optimization_level%': '2',
   },
   'target_defaults': {
     'configurations': {
       'Dart_Base': {
         'cflags': [
           '-Werror',
           '<@(common_gcc_warning_flags)',
           '-Wnon-virtual-dtor',
           '-Wvla',
           '-Wno-conversion-null',
@@ skipping 35 matching lines @@
           '-mfpu=vfp',
           '-mfloat-abi=softfp',
           '-fno-strict-overflow',
         ],
         'ldflags': [
           '-Wl,-rpath=<(arm_cross_libc)/usr/lib',
         ],
       },
 
       'Dart_Debug': {
-        'cflags': [ '-O2' ],
+        'cflags': [ '-O<(dart_debug_optimization_level)' ],
       },
 
       'Dart_Release': {
         'cflags': [ '-O3', ],
       },
     },
   },
 }