Check for COVERALLS_TOKEN before trying running coverall
diff --git a/build.sh b/build.sh
index f655354..37a8bae 100755
--- a/build.sh
+++ b/build.sh
@@ -20,7 +20,7 @@
 pub run test
 
 # Install dart_coveralls; gather and send coverage data.
-if [ "$TRAVIS_DART_VERSION" = "stable" ]; then
+if [ "$COVERALLS_TOKEN" ] && [ "$TRAVIS_DART_VERSION" = "stable" ]; then
   pub global activate dart_coveralls
   pub global run dart_coveralls report \
     --retry 2 \