Revert "Fix scala-js#3154: Avoid calling AssertionError(String, Throwable)."

This reverts commit 9428b68.

That commit was avoiding a constructor not available on JDK6. Now
that we only build on JDK8+, we can use the `(String, Throwable)`
constructor, which is cleaner.

Author: sjrd

junit-runtime/src/main/scala/org/junit/internal/ArrayComparisonFailure.scala

@@ -5,14 +5,8 @@ package org.junit.internal
 
 object ArrayComparisonFailure
 
-/* Attention: AssertionError does not have a (String, Throwable) constructor
- * in JDK 6. If we try to call that one, compilation *succeeds* because of
- * auto-boxing, but it does not do the right thing. See #3154.
- */
 class ArrayComparisonFailure(message: String, cause: AssertionError, index: Int)
-    extends AssertionError(message) {
-
-  initCause(cause)
+    extends AssertionError(message, cause) {
 
   private var fIndices: List[Int] = index :: Nil
 
@@ -29,7 +23,7 @@ class ArrayComparisonFailure(message: String, cause: AssertionError, index: Int)
     val indices =
       if (fIndices == null) s"[$index]" // see #3148
       else fIndices.map(index => s"[$index]").mkString
-    val causeMessage = cause.getMessage // do not use getCause(), see #3148
+    val causeMessage = getCause.getMessage
     s"${msg}arrays first differed at element $indices; $causeMessage"
   }