Fixed ThreadSafeHeapTest for JDK 1.6
diff --git a/kotlinx-coroutines-core/src/test/kotlin/kotlinx/coroutines/experimental/internal/ThreadSafeHeapTest.kt b/kotlinx-coroutines-core/src/test/kotlin/kotlinx/coroutines/experimental/internal/ThreadSafeHeapTest.kt
index 91257e1..2d576ab 100644
--- a/kotlinx-coroutines-core/src/test/kotlin/kotlinx/coroutines/experimental/internal/ThreadSafeHeapTest.kt
+++ b/kotlinx-coroutines-core/src/test/kotlin/kotlinx/coroutines/experimental/internal/ThreadSafeHeapTest.kt
@@ -25,7 +25,7 @@
 class ThreadSafeHeapTest {
     class Node(val value: Int) : ThreadSafeHeapNode, Comparable<Node> {
         override var index = -1
-        override fun compareTo(other: Node): Int = Integer.compare(value, other.value)
+        override fun compareTo(other: Node): Int = value.compareTo(other.value)
         override fun equals(other: Any?): Boolean = other is Node && other.value == value
         override fun hashCode(): Int = value
         override fun toString(): String = "$value"