Adds dart_debug parameter to GN build.

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.
 
+# TODO(zra): These build arguments should likely be moved to a gni file that is
+# included in BUILD.gn files that care about the values of the flags. For now,
+# since the GN build only happens as part of a Mojo build there is no need for
+# the indirection.
+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
+}
+
 config("dart_config") {
   defines = []
-  if (is_debug) {
+  if (dart_debug) {
     defines += ["DEBUG"]
   } else {
     defines += ["NDEBUG"]
   }
 
   cflags = [
     "-Werror",
     "-Wall",
     "-Wextra", # Also known as -W.
     "-Wno-unused-parameter",
     "-Wnon-virtual-dtor",
     "-Wvla",
     "-Wno-conversion-null",
     "-Woverloaded-virtual",
     "-g3",
     "-ggdb3",
     "-fno-rtti",
     "-fno-exceptions",
   ]
+
+  if (dart_debug) {
+    cflags += [
+      "-O1",
+    ]
+  } else {
+    cflags += [
+      "-O3",
+    ]
+  }
 }
 
 
 static_library("libdart") {
   configs += [":dart_config"]
   deps = [
     "vm:libdart_lib",
     "vm:libdart_vm",
     "third_party/jscre:libjscre",
     "third_party/double-conversion/src:libdouble_conversion",
@@ skipping 38 matching lines @@
   args = [
     "--quiet",
     "--output", rebase_path(output, root_build_dir),
     "--input", rebase_path("vm/version_in.cc", root_build_dir),
     "--ignore_svn_revision",
   ]
 }
 
 
 executable("libdart_dependency_helper") {
+  configs += [":dart_config"]
   deps = [
     "vm:libdart_lib_withcore",
     "vm:libdart_lib",
     "vm:libdart_vm",
     "vm:libdart_platform",
     "third_party/jscre:libjscre",
     "third_party/double-conversion/src:libdouble_conversion",
   ]
   sources = [
     "vm/libdart_dependency_helper.cc",
   ]
 }