Allow pull requests to skip an implementation.

.travis.yml

@@ -26,6 +26,7 @@ matrix:
     - env: LANGUAGE_VERSION=4.0 IMPL=dart-sass
 
 before_install:
+  - if ./tools/skipped-for-impl.sh; then exit 0; fi
   - rm Gemfile.lock
 
   - if [ $IMPL == "libsass" ]; then

tools/skipped-for-impl.sh

@@ -0,0 +1,26 @@
+#!/bin/bash -e
+
+# Exits with exit code 0 if the tests should be skipped for the current
+# implementation, and exit code 1 otherwise. This allows pull requests to
+# include e.g. "Skip dart-sass" if a connected Dart Sass pull request is also in
+# flight. Multiple implementations can also be skipped: "Skip libsass,
+# ruby-sass".
+
+if [ "$TRAVIS_PULL_REQUEST" == "false" ]; then
+  >&2 echo "TRAVIS_PULL_REQUEST: $TRAVIS_PULL_REQUEST."
+  >&2 echo "Not skipping."
+  exit 1
+fi
+
+>&2 echo "Fetching pull request $TRAVIS_PULL_REQUEST..."
+
+JSON=$(curl -L -sS https://api.github.com/repos/sass/sass-spec/pulls/$TRAVIS_PULL_REQUEST)
+
+RE_SKIP="[Ss]kip\>[a-z, -]*\<$IMPL"
+
+if [[ $JSON =~ $RE_SPEC_PR && "${BASH_REMATCH[2]}" == "$IMPL" ]]; then
+  >&2 echo "Skipping $IMPL."
+  exit 0
+else
+  exit 1
+fi