Make the debug optimization level configurable through a variable.

tools/gyp/configurations_msvs.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_Debug': {
         'msvs_settings': {
           'VCCLCompilerTool': {
-            'Optimization': '2',
+            'Optimization': '<(dart_debug_optimization_level)',
             'BasicRuntimeChecks': '0',  # disable /RTC1 when compiling /O2
             'DebugInformationFormat': '3',
             'ExceptionHandling': '0',
             'RuntimeTypeInfo': 'false',
             'OmitFramePointers': 'false',
           },
           'VCLinkerTool': {
             'LinkIncremental': '2',
             'GenerateDebugInformation': 'true',
             'StackReserveSize': '2097152',
@@ skipping 18 matching lines @@
             'GenerateDebugInformation': 'true',
             'OptimizeReferences': '2',
             'EnableCOMDATFolding': '2',
             'StackReserveSize': '2097152',
           },
         },
       },
     },
   },
 }