flutter pub update & various fixes (#627)

infinite_list/android/build.gradle

@@ -1,4 +1,5 @@
 buildscript {
+    ext.kotlin_version = '1.3.50'
     repositories {
         google()
         jcenter()
@@ -6,6 +7,7 @@ buildscript {
 
     dependencies {
         classpath 'com.android.tools.build:gradle:3.5.0'
+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
     }
 }