SPARK-1488. Resolve scalac feature warnings during build

For your consideration: scalac currently notes a number of feature warnings during compilation:

```
[warn] there were 65 feature warning(s); re-run with -feature for details
```

Warnings are like:

```
[warn] /Users/srowen/Documents/spark/core/src/main/scala/org/apache/spark/SparkContext.scala:1261: implicit conversion method rddToPairRDDFunctions should be enabled
[warn] by making the implicit value scala.language.implicitConversions visible.
[warn] This can be achieved by adding the import clause 'import scala.language.implicitConversions'
[warn] or by setting the compiler option -language:implicitConversions.
[warn] See the Scala docs for value scala.language.implicitConversions for a discussion
[warn] why the feature should be explicitly enabled.
[warn]   implicit def rddToPairRDDFunctions[K: ClassTag, V: ClassTag](rdd: RDD[(K, V)]) =
[warn]                ^
```

scalac is suggesting that it's just best practice to explicitly enable certain language features by importing them where used.

This PR simply adds the imports it suggests (and squashes one other Java warning along the way). This leaves just deprecation warnings in the build.

Author: Sean Owen <sowen@cloudera.com>

Closes #404 from srowen/SPARK-1488 and squashes the following commits:

8598980 [Sean Owen] Quiet scalac warnings about language features by explicitly importing language features.
39bc831 [Sean Owen] Enable -feature in scalac to emit language feature warnings

8 files changed, 9 insertions, 0 deletions

diff --git a/streaming/src/main/scala/org/apache/spark/streaming/StreamingContext.scala b/streaming/src/main/scala/org/apache/spark/streaming/StreamingContext.scala
index ff5d0aaa3d..e9a4f7ba22 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/StreamingContext.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/StreamingContext.scala
@@ -22,6 +22,7 @@ import java.util.concurrent.atomic.AtomicInteger
 
 import scala.collection.Map
 import scala.collection.mutable.Queue
+import scala.language.implicitConversions
 import scala.reflect.ClassTag
 
 import akka.actor.{Props, SupervisorStrategy}
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaDStream.scala b/streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaDStream.scala
index 721d502732..13e2bacc92 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaDStream.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaDStream.scala
@@ -23,6 +23,7 @@ import org.apache.spark.api.java.JavaRDD
 import org.apache.spark.storage.StorageLevel
 import org.apache.spark.rdd.RDD
 
+import scala.language.implicitConversions
 import scala.reflect.ClassTag
 import org.apache.spark.streaming.dstream.DStream
 
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaDStreamLike.scala b/streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaDStreamLike.scala
index bb2f492d06..a6184de4e8 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaDStreamLike.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaDStreamLike.scala
@@ -22,6 +22,7 @@ import java.lang.{Long => JLong}
 import java.util.{List => JList}
 
 import scala.collection.JavaConversions._
+import scala.language.implicitConversions
 import scala.reflect.ClassTag
 
 import org.apache.spark.api.java.{JavaPairRDD, JavaRDD, JavaRDDLike}
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaPairDStream.scala b/streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaPairDStream.scala
index 2ac943d7bf..cb8e8f00a7 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaPairDStream.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaPairDStream.scala
@@ -21,6 +21,7 @@ import java.lang.{Long => JLong, Iterable => JIterable}
 import java.util.{List => JList}
 
 import scala.collection.JavaConversions._
+import scala.language.implicitConversions
 import scala.reflect.ClassTag
 
 import com.google.common.base.Optional
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/receivers/ActorReceiver.scala b/streaming/src/main/scala/org/apache/spark/streaming/receivers/ActorReceiver.scala
index f5984d03c5..da0d364ae7 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/receivers/ActorReceiver.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/receivers/ActorReceiver.scala
@@ -23,6 +23,7 @@ import akka.actor.{ PossiblyHarmful, OneForOneStrategy }
 import akka.actor.SupervisorStrategy._
 
 import scala.concurrent.duration._
+import scala.language.postfixOps
 import scala.reflect.ClassTag
 
 import org.apache.spark.storage.{StorageLevel, StreamBlockId}
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/scheduler/NetworkInputTracker.scala b/streaming/src/main/scala/org/apache/spark/streaming/scheduler/NetworkInputTracker.scala
index a1e6f51768..438e72a7ce 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/scheduler/NetworkInputTracker.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/scheduler/NetworkInputTracker.scala
@@ -18,6 +18,7 @@
 package org.apache.spark.streaming.scheduler
 
 import scala.collection.mutable.{HashMap, SynchronizedMap, SynchronizedQueue}
+import scala.language.existentials
 
 import akka.actor._
 
diff --git a/streaming/src/test/scala/org/apache/spark/streaming/StreamingContextSuite.scala b/streaming/src/test/scala/org/apache/spark/streaming/StreamingContextSuite.scala
index efd0d22ecb..ad5367ab94 100644
--- a/streaming/src/test/scala/org/apache/spark/streaming/StreamingContextSuite.scala
+++ b/streaming/src/test/scala/org/apache/spark/streaming/StreamingContextSuite.scala
@@ -28,6 +28,8 @@ import org.scalatest.concurrent.Timeouts
 import org.scalatest.exceptions.TestFailedDueToTimeoutException
 import org.scalatest.time.SpanSugar._
 
+import scala.language.postfixOps
+
 class StreamingContextSuite extends FunSuite with BeforeAndAfter with Timeouts with Logging {
 
   val master = "local[2]"
diff --git a/streaming/src/test/scala/org/apache/spark/streaming/UISuite.scala b/streaming/src/test/scala/org/apache/spark/streaming/UISuite.scala
index 35538ec188..031e93ab24 100644
--- a/streaming/src/test/scala/org/apache/spark/streaming/UISuite.scala
+++ b/streaming/src/test/scala/org/apache/spark/streaming/UISuite.scala
@@ -18,6 +18,7 @@
 package org.apache.spark.streaming
 
 import scala.io.Source
+import scala.language.postfixOps
 
 import org.scalatest.FunSuite
 import org.scalatest.concurrent.Eventually._