Implement common Throwable.addSuppressed/suppressed extensions

Instead of `suppressed` Array we provide `suppressedExceptions` List

#KT-23737

libraries/stdlib/js/src/kotlin/throwableExtensions.kt

@@ -0,0 +1,31 @@
+/*
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package kotlin
+
+/**
+ * Adds the specified exception to the list of exceptions that were
+ * suppressed in order to deliver this exception.
+ */
+@SinceKotlin("1.4")
+public actual fun Throwable.addSuppressed(exception: Throwable) {
+    if (this !== exception) {
+        val suppressed = this.asDynamic()._suppressed.unsafeCast<MutableList<Throwable>?>()
+        if (suppressed == null) {
+            this.asDynamic()._suppressed = mutableListOf(exception)
+        } else {
+            suppressed.add(exception)
+        }
+    }
+}
+
+/**
+ * Returns a list of all exceptions that were suppressed in order to deliver this exception.
+ */
+@SinceKotlin("1.4")
+public actual val Throwable.suppressedExceptions: List<Throwable>
+    get() {
+        return this.asDynamic()._suppressed?.unsafeCast<List<Throwable>>() ?: emptyList()
+    }

libraries/stdlib/js/test/core/testUtils.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -23,4 +23,6 @@ public actual fun testOnJvm(action: () -> Unit) { }
 public actual fun testOnJs(action: () -> Unit) = action()
 
 // TODO: should be true at least in JS IR after implementing KT-24975
-public actual val isFloat32RangeEnforced: Boolean = false
+public actual val isFloat32RangeEnforced: Boolean = false
+
+actual val supportsSuppressedExceptions: Boolean get() = true