Fix bash conditional for platform detection

Bug: 35993037
Test: ./gradlew createArchive
Change-Id: I5adbb8b808200511b15752a1d07e4cd05fe9ed13
diff --git a/gradlew b/gradlew
index 98b9f82..4c2ef38 100755
--- a/gradlew
+++ b/gradlew
@@ -62,7 +62,7 @@
 CLASSPATH=$APP_HOME/gradle/wrapper/gradle-wrapper.jar
 
 # Pick the correct fullsdk for this OS.
-if [ darwin ]; then
+if [ $darwin == "true" ]; then
     plat="darwin"
 else
     plat="linux"