Null-aware types introduced

Flexible types should drive their own conversions to nullable/not-null

compiler/testData/diagnostics/tests/platformTypes/commonSupertype/collectionOrNull.kt

@@ -0,0 +1,19 @@
+// FILE: p/Utils.java
+
+package p;
+
+public class Utils {
+    public static java.util.Collection<String> c() { return null; }
+}
+
+// FILE: k.kt
+
+import p.*
+
+fun <T : Any> T.foo() {}
+
+fun test<D>(b: Boolean) {
+    val c = if (b) Utils.c() else null
+
+    c?.foo()
+}

compiler/testData/diagnostics/tests/platformTypes/commonSupertype/stringOrNull.kt

@@ -0,0 +1,19 @@
+// FILE: p/Utils.java
+
+package p;
+
+public class Utils {
+    public static String str() { return null; }
+}
+
+// FILE: k.kt
+
+import p.*
+
+fun <T : Any> T.foo() {}
+
+fun test<D>(b: Boolean) {
+    val str = if (b) Utils.str() else null
+
+    str?.foo()
+}