Enable Dart's checked mode in the Sky controller by default for debug builds

sky/engine/config.gni

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/ui.gni")
 
 if (is_android) {
   import("//build/config/android/config.gni")
 }
 
 if (target_cpu == "arm") {
   import("//build/config/arm.gni")
 } else {
   # TODO(brettw) remove this once && early-out is checked in.
   arm_version = 0
 }
 
 declare_args() {
   # Enable asserts, even in release builds.
   sky_asserts_always_on = false
 
   # Experimental support for the Dart VM.
   sky_use_dart = false
+
+  # Default to strict mode in debug builds.
+  if (is_debug) {
+    sky_dart_strict_mode = true
+  } else {
+    sky_dart_strict_mode = false
+  }
 }
 
 # feature_defines_list ---------------------------------------------------------
 
 feature_defines_list = []
 
 if (is_debug) {
   # TODO(GYP) this should also be set when release_valgrind_build is set,
   # but we don't have that flag in the GN build yet.
   feature_defines_list += [ "WTF_USE_DYNAMIC_ANNOTATIONS=1" ]
 }
 
 if (!is_mac) {
   # Mac OS X has not implemented support for ENABLE(OPENTYPE_VERTICAL) yet.
   feature_defines_list += [ "ENABLE_OPENTYPE_VERTICAL=1" ]
 }
 
 if (sky_asserts_always_on) {
   feature_defines_list += [ "ENABLE_ASSERT=1" ]
 }
 
+if (sky_dart_strict_mode) {
+  feature_defines_list += [ "ENABLE_DART_STRICT=1" ]
+}
+
 if (sky_use_dart) {
   feature_defines_list += [ "WTF_USE_DART=1" ]
 }
 
 # feature_defines_string -------------------------------------------------------
 
 # Convert the list to a space-separated string for passing to scripts.
 # This would be the equivalent of passing '<(feature_defines)' in GYP.
 feature_defines_string =
     exec_script("build/gn_list_to_space_separated_string.py",
                 feature_defines_list,
                 "trim string")