hardcode Map#get as having unknown nullity (IDEA-176626)

until we have a better solution

java/java-tests/testData/inspection/dataFlow/fixture/MapGetWithNotNullKeys.java

@@ -0,0 +1,17 @@
+import foo.*;
+import java.util.*;
+
+class Test {
+
+  public void main(Map<@NotNull String, @NotNull Integer> map, HashMap<@NotNull String, @NotNull Integer> hashMap) {
+    Integer value = map.get("y");
+    if (value == null) {
+      System.out.println("it's not contained");
+    }
+
+    value = hashMap.get("y");
+    if (value == null) {
+      System.out.println("it's not contained");
+    }
+  }
+}

java/java-tests/testSrc/com/intellij/java/codeInspection/DataFlowInspection8Test.java

@@ -197,6 +197,8 @@ public class DataFlowInspection8Test extends DataFlowInspectionTestCase {
   public void testStreamInlining() { doTest(); }
   public void testStreamComparatorInlining() { doTest(); }
   public void testStreamKnownSource() { doTest(); }
+  
+  public void testMapGetWithNotNullKeys() { doTestWithCustomAnnotations(); }
 
   public void testMethodVsExpressionTypeAnnotationConflict() {
     setupAmbiguousAnnotations("withTypeUse", myFixture);