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

sky/engine/config.gni

Index: sky/engine/config.gni
diff --git a/sky/engine/config.gni b/sky/engine/config.gni
index d6214be92fa4814614b0c3b5f3aebd7dfbb51b29..47a51ef00cbe9a3ec1ef88887234c8a3d3de8081 100644
--- a/sky/engine/config.gni
+++ b/sky/engine/config.gni
@@ -21,6 +21,13 @@ declare_args() {
 
   # 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 ---------------------------------------------------------
@@ -42,6 +49,10 @@ 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" ]
 }