fixed robolectric

android/app/build.gradle

 apply plugin: 'com.android.library'
+apply from: '../prepare-robolectric.gradle'
 
 android {
     compileSdkVersion 23

android/prepare-robolectric.gradle

+def robolectricDependenciesFolder = rootProject.buildDir.path + "/robolectric-dependencies"
+
+configurations.create('robolectricRuntime')
+
+dependencies {
+    robolectricRuntime "org.ccil.cowan.tagsoup:tagsoup:1.2"
+    robolectricRuntime "org.robolectric:android-all:6.0.0_r1-robolectric-0"
+    robolectricRuntime "org.robolectric:shadows-core-v23:3.1.4"
+    robolectricRuntime "org.json:json:20080701"
+}
+
+rootProject.task(type: Copy, overwrite: true, "downloadRobolectricDependencies") {
+    println "downloadRobolectricDependencies " + robolectricDependenciesFolder
+    from configurations.robolectricRuntime
+    into robolectricDependenciesFolder
+}
+
+project.afterEvaluate {
+  tasks.all {
+      if (it.name.startsWith("test")) {
+          it.dependsOn(rootProject.tasks.findByName("downloadRobolectricDependencies"))
+      }
+  }
+}
+
+android {
+    testOptions {
+        unitTests.all {
+            systemProperty 'robolectric.offline', 'true'
+            systemProperty 'robolectric.dependency.dir', robolectricDependenciesFolder
+        }
+    }
+}
+

package.json

     "build": ":",
     "lint": "eslint src test",
     "test:js": "jest",
-    "test:android": "cd android && ./gradlew clean testDebugUnitTest -Dpre-dex=false",
+    "test:android": "cd android && ./gradlew clean testDebugUnitTest -Dpre-dex=true",
     "test:ios": ":",
     "pretest": "npm run lint",
     "test": "npm run test:js && npm run test:android && npm run test:ios",