Make the debug optimization level configurable through a variable.

tools/gyp/configurations_xcode.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': {
+    'dart_debug_optimization_level%': '2',
+  },
   'target_defaults': {
     'configurations': {
       'Dart_Base': {
         'xcode_settings': {
           # To switch to the LLVM based backend change the two lines below.
           #'GCC_VERSION': 'com.apple.compilers.llvmgcc42',
           'GCC_VERSION': '4.2',
           'GCC_C_LANGUAGE_STANDARD': 'ansi',
           'GCC_ENABLE_CPP_EXCEPTIONS': 'NO', # -fno-exceptions
           'GCC_ENABLE_CPP_RTTI': 'NO', # -fno-rtti
@@ skipping 44 matching lines @@
 
       'Dart_simarm_Base': {
         'xcode_settings': {
           'ARCHS': [ 'i386' ],
           'GCC_OPTIMIZATION_LEVEL': '3',
         },
       },
 
       'Dart_Debug': {
         'xcode_settings': {
-          'GCC_OPTIMIZATION_LEVEL': '2',
+          'GCC_OPTIMIZATION_LEVEL': '<(dart_debug_optimization_level)',
         },
       },
 
       'Dart_Release': {
         'xcode_settings': {
           'GCC_OPTIMIZATION_LEVEL': '3',
         },
       },
     },
   },
 }