Implement common Throwable.addSuppressed/suppressed extensions

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

#KT-23737

libraries/stdlib/jdk7/src/kotlin/internal/jdk7/JDK7PlatformImplementations.kt

@@ -1,17 +1,6 @@
 /*
- * Copyright 2010-2017 JetBrains s.r.o.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
+ * 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.
  */
 
 @file:Suppress("INVISIBLE_REFERENCE", "INVISIBLE_MEMBER", "CANNOT_OVERRIDE_INVISIBLE_MEMBER")
@@ -23,4 +12,5 @@ internal open class JDK7PlatformImplementations : PlatformImplementations() {
 
     override fun addSuppressed(cause: Throwable, exception: Throwable) = cause.addSuppressed(exception)
 
+    override fun getSuppressed(exception: Throwable): List<Throwable> = exception.suppressed.asList()
 }

libraries/stdlib/jdk7/testNoJdk7/ExceptionTest.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.
  */
 
@@ -15,9 +15,11 @@ class ExceptionTest {
         val e1 = Throwable()
         val e2 = Exception("Suppressed")
 
+        assertTrue(e1.suppressedExceptions.isEmpty())
         e1.addSuppressedExtension(e2)
 
         assertSame(e2, e1.suppressed.singleOrNull())
+        assertSame(e2, e1.suppressedExceptions.singleOrNull())
     }
 
 }