Add RedundantImport check and upgrade checkstyle to 8.0. (#579)

Author: Bogdan Drutu

build.gradle

@@ -164,7 +164,7 @@ subprojects {
 
     checkstyle {
         configFile = file("$rootDir/buildscripts/checkstyle.xml")
-        toolVersion = "7.6"
+        toolVersion = "8.0"
         ignoreFailures = false
         if (rootProject.hasProperty("checkstyle.ignoreFailures")) {
             ignoreFailures = rootProject.properties["checkstyle.ignoreFailures"].toBoolean()

buildscripts/checkstyle.xml

@@ -50,6 +50,7 @@
             <property name="ignorePattern" value="^package.*|^import.*|a href|href|http://|https://|ftp://"/>
         </module>
         <module name="AvoidStarImport"/>
+        <module name="RedundantImport"/>
         <module name="OneTopLevelClass"/>
         <module name="NoLineWrap"/>
         <module name="EmptyBlock">

impl/src/test/java/io/opencensus/impl/internal/DisruptorEventQueueTest.java

@@ -18,7 +18,6 @@
 
 import static com.google.common.truth.Truth.assertThat;
 
-import io.opencensus.impl.internal.DisruptorEventQueue;
 import io.opencensus.implcore.internal.EventQueue;
 import org.junit.Test;
 import org.junit.runner.RunWith;

impl_core/src/test/java/io/opencensus/implcore/internal/TimestampConverterTest.java

@@ -21,7 +21,6 @@
 
 import io.opencensus.common.Clock;
 import io.opencensus.common.Timestamp;
-import io.opencensus.implcore.internal.TimestampConverter;
 import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;

impl_core/src/test/java/io/opencensus/implcore/trace/config/TraceConfigImplTest.java

@@ -18,7 +18,6 @@
 
 import static com.google.common.truth.Truth.assertThat;
 
-import io.opencensus.implcore.trace.config.TraceConfigImpl;
 import io.opencensus.trace.config.TraceParams;
 import io.opencensus.trace.samplers.Samplers;
 import org.junit.Test;

impl_core/src/test/java/io/opencensus/implcore/trace/internal/ConcurrentIntrusiveListTest.java

@@ -18,7 +18,6 @@
 
 import static com.google.common.truth.Truth.assertThat;
 
-import io.opencensus.implcore.trace.internal.ConcurrentIntrusiveList;
 import io.opencensus.implcore.trace.internal.ConcurrentIntrusiveList.Element;
 import org.junit.Rule;
 import org.junit.Test;

impl_lite/src/test/java/io/opencensus/impllite/trace/TraceComponentImplLiteTest.java

@@ -22,7 +22,6 @@
 import io.opencensus.implcore.trace.TracerImpl;
 import io.opencensus.implcore.trace.export.ExportComponentImpl;
 import io.opencensus.implcore.trace.propagation.PropagationComponentImpl;
-import io.opencensus.impllite.trace.TraceComponentImplLite;
 import io.opencensus.trace.Tracing;
 import org.junit.Test;
 import org.junit.runner.RunWith;