Add test for contracts

backend.native/tests/build.gradle

@@ -1183,6 +1183,11 @@ task classDelegation_linkTest(type: LinkKonanTest) {
     lib = "codegen/classDelegation/linkTest_lib.kt"
 }
 
+task contracts(type: RunStandaloneKonanTest) {
+    flags = [ '-Xuse-experimental=kotlin.Experimental', '-tr' ]
+    source = "codegen/contracts/contracts.kt"
+}
+
 task delegatedProperty_simpleVal(type: RunKonanTest) {
     goldValue = "x\n42\n"
     source = "codegen/delegatedProperty/simpleVal.kt"

backend.native/tests/codegen/contracts/contracts.kt

@@ -0,0 +1,47 @@
+import kotlin.test.*
+import kotlin.contracts.*
+
+open class S
+class P(val str: String = "P") : S()
+
+@UseExperimental(kotlin.contracts.ExperimentalContracts::class)
+fun check(actual: Boolean) {
+    contract { returns() implies actual }
+    assertTrue(actual)
+}
+
+@Test fun testContractForCast() {
+    val s: S = P()
+
+    check(s is P)
+    assertEquals(s.str, "P")
+}
+
+@Test fun testRequire() {
+    val s: S = P()
+
+    require(s is P)
+    assertEquals(s.str, "P")
+}
+
+@Test fun testNonNullSmartCast() {
+    val i: Int? = 1234
+    requireNotNull(i)
+    assertEquals(i, 1234)
+}
+
+@Test fun testRunLambdaForVal() {
+    val x: Int
+    run {
+        x = 42
+    }
+    assertEquals(x, 42)
+}
+
+@Test fun testIsNullString() {
+    assertEquals("STR", nullableString("str"))
+
+    assertEquals("", nullableString(null))
+}
+
+private fun nullableString(string: String?): String = if (string.isNullOrBlank()) "" else "STR"