title: Unresolved reference async in Kotlin
body: <p>I am trying to perform network operation async in <code>Kotlin</code>. I read it you can do async using <code>async</code> function. I am getting below error, can anyone guess what could be the issue ?</p>
  
  <blockquote>
    <p>Unresolved reference: async</p>
  </blockquote>
  
  <pre><code>override fun onCreate(savedInstanceState: Bundle?) {
      super.onCreate(savedInstanceState)
      setContentView(R.layout.activity_main)
  
      async() {
          val forecastWeather = ForecastRequest("302015").execute()
          Log.d("Test", forecastWeather.toString())
      }
  }
  </code></pre>
  
  <p><strong>ForecastRequest.kt</strong></p>
  
  <pre><code>class ForecastRequest(val zipcode: String) {
      companion object {
          private val APP_ID = "XYZ"
          private val URL = "http://api.openweathermap.org/data/2.5/" +
                      "forecast/daily?mode=json&amp;units=metric&amp;cnt=7"
          private val COMPLETE_URL = "$URL&amp;APPID=$APP_ID&amp;q="
      }
  
      fun execute(): ForecastResponse {
          val forecastJsonStr = java.net.URL(COMPLETE_URL + zipcode).readText()
          return Gson().fromJson(forecastJsonStr, ForecastResponse::class.java)
      }
  }
  </code></pre>
  
  <p>Top level <strong>build.gradle</strong></p>
  
  <pre><code>// Top-level build file where you can add configuration options common to all sub-projects/modules.
  
  buildscript {
      ext.kotlin_version = '1.1.2-4'
      repositories {
          maven { url 'https://maven.google.com' }
          jcenter()
      }
      dependencies {
          classpath 'com.android.tools.build:gradle:3.0.0-alpha2'
          classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
  
          // NOTE: Do not place your application dependencies here; they belong
          // in the individual module build.gradle files
      }
  }
  
  allprojects {
      repositories {
          maven { url 'https://maven.google.com' }
          jcenter()
          mavenCentral()
      }
  }
  
  task clean(type: Delete) {
      delete rootProject.buildDir
  }
  </code></pre>
  
  <p>Module level <strong>build.gradle</strong></p>
  
  <pre><code>apply plugin: 'com.android.application'
  
  apply plugin: 'kotlin-android'
  
  android {
      compileSdkVersion 25
      buildToolsVersion "25.0.2"
      defaultConfig {
          applicationId "com.williams.thirddemo"
          minSdkVersion 23
          targetSdkVersion 25
          versionCode 1
          versionName "1.0"
          testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
      }
      buildTypes {
          release {
              minifyEnabled false
              proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
          }
      }
  }
  
  dependencies {
      compile fileTree(dir: 'libs', include: ['*.jar'])
      androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
          exclude group: 'com.android.support', module: 'support-annotations'
      })
      compile "org.jetbrains.kotlin:kotlin-stdlib-jre7:$kotlin_version"
      compile 'com.android.support:appcompat-v7:25.3.1'
      testCompile 'junit:junit:4.12'
      compile 'com.android.support.constraint:constraint-layout:1.0.2'
      compile 'com.google.code.gson:gson:2.8.1'
  }
  </code></pre>