Make pkg/analysis_server/tool/spec/generate_files work on Linux

We have comments in some generated files suggesting that the user use
this script to regenerate them, but the script previously only worked
on Macs. This CL fixes the script so that it work with Linux too.

Change-Id: I4b42b843513247b16bbbcd0b7c85f82d6a6a6964
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/507540
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

pkg/analysis_server/tool/spec/generate_files

@@ -37,6 +37,8 @@ DART_CONFIGURATION="ReleaseX64"
 if [[ `uname` == 'Darwin' ]];
 then
   BUILD_DIR="${ROOT_DIR}/xcodebuild/$DART_CONFIGURATION"
+else
+  BUILD_DIR="${ROOT_DIR}/out/$DART_CONFIGURATION"
 fi
 
 PKG_FILE="${ROOT_DIR}/.dart_tool/package_config.json"