Move Dart SDK to 1.15-dev.4 to support _embedder.yaml

bin/cache/update_dart_sdk.sh

diff --git a/bin/cache/update_dart_sdk.sh b/bin/cache/update_dart_sdk.sh
index 2e29944..a2a40f6 100755
--- a/bin/cache/update_dart_sdk.sh
+++ b/bin/cache/update_dart_sdk.sh
@@ -27,7 +27,14 @@
       ;;
   esac
 
-  DART_SDK_URL="http://storage.googleapis.com/dart-archive/channels/stable/raw/$DART_SDK_VERSION/sdk/$DART_ZIP_NAME"
+  DART_CHANNEL="stable"
+
+  if [[ $DART_SDK_VERSION == *"-dev."* ]]
+  then
+    DART_CHANNEL="dev"
+  fi
+
+  DART_SDK_URL="http://storage.googleapis.com/dart-archive/channels/$DART_CHANNEL/raw/$DART_SDK_VERSION/sdk/$DART_ZIP_NAME"
 
   rm -rf -- "$DART_SDK_PATH"
   mkdir -p -- "$DART_SDK_PATH"