Tests for recently fixed bugs

compiler/testData/checkerWithErrorTypes/quick/regressions/kt174.jet

@@ -0,0 +1,8 @@
+// KT-174 Nullability info for extension function receivers
+// +JDK
+trait Tree {}
+
+fun Any?.TreeValue() : Tree {
+  if (this is Tree) return this
+    throw Exception()
+}

compiler/testData/checkerWithErrorTypes/quick/regressions/kt201.jet

@@ -0,0 +1,9 @@
+// KT-201 Allow to call extension with nullable receiver with a '.'
+// +JDK
+
+fun <T : Any> T?.npe() : T = if (this == null) throw NullPointerException() else this
+
+fun foo() {
+  val i : Int? = 1
+  i.npe() // error!
+}

compiler/testData/checkerWithErrorTypes/quick/regressions/kt312.jet

@@ -0,0 +1,10 @@
+// KT-312 Nullability problem when a nullable version of a generic type is returned
+
+fun <T> Array<out T>.safeGet(index : Int) : T? {
+    return if (index < size) this[index] else null
+}
+
+val args : Array<String> = Array<String>(1)
+val name : String = <!TYPE_MISMATCH!>args.safeGet<String>(0)<!> // No error, must be type mismatch
+val name1 : String? = args.safeGet(0)
+