Use https instead of http when downloading Dart. (#4821)

This came up in security review.

@abarth

bin/cache/update_dart_sdk.sh

@@ -34,7 +34,7 @@ if [ ! -f "$DART_SDK_STAMP_PATH" ] || [ "$DART_SDK_VERSION" != `cat "$DART_SDK_S
     DART_CHANNEL="dev"
   fi
 
-  DART_SDK_URL="http://storage.googleapis.com/dart-archive/channels/$DART_CHANNEL/raw/$DART_SDK_VERSION/sdk/$DART_ZIP_NAME"
+  DART_SDK_URL="https://storage.googleapis.com/dart-archive/channels/$DART_CHANNEL/raw/$DART_SDK_VERSION/sdk/$DART_ZIP_NAME"
 
   rm -rf -- "$DART_SDK_PATH"
   mkdir -p -- "$DART_SDK_PATH"