Add support for configurable -OX optimization levels for debug builds

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 10 matching lines @@
   # of the Dart VM should leave this set to "develop", and should set
   # 'is_debug', 'is_release', or 'is_product'.
   dart_runtime_mode = "develop"
 
   # Explicitly set the target architecture in case of precompilation. Leaving
   # this unspecified results in automatic target architecture detection.
   # Available options are: arm, arm64, mips, x64 and ia32
   dart_target_arch = ""
 
   dart_experimental_interpreter = false
+
+  # The optimization level to use for debug builds.
+  dart_debug_optimization_level = "2"
 }
 
 config("dart_public_config") {
   include_dirs = [ "." ]
 }
 
 # Adds PRODUCT define if Flutter has specified "release" for dart_runtime_mode
 config("dart_maybe_product_config") {
   defines = []
 
@@ skipping 115 matching lines @@
       "-Wvla",
       "-Wno-conversion-null",
       "-Woverloaded-virtual",
       "-g3",
       "-ggdb3",
       "-fno-rtti",
       "-fno-exceptions",
     ]
 
     if (dart_debug) {
-      cflags += [ "-O2" ]
+      cflags += [ "-O${dart_debug_optimization_level}" ]
     } else {
       cflags += [ "-O3" ]
     }
 
     ldflags = []
     if (defined(is_asan) && is_asan) {
       ldflags += [ "-fsanitize=address" ]
     }
     if (defined(is_msan) && is_msan) {
       ldflags += [ "-fsanitize=memory" ]
@@ skipping 113 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",
   ]
 }