8170945: Collectors$Partition should override more Map methods

Reviewed-by: smarks

src/java.base/share/classes/java/util/stream/Collectors.java

@@ -1972,6 +1972,35 @@ public final class Collectors {
             this.forFalse = forFalse;
         }
 
+        @Override
+        public int size() {
+            return 2;
+        }
+
+        @Override
+        public boolean isEmpty() {
+            return false;
+        }
+
+        @Override
+        public T get(Object key) {
+            if (key instanceof Boolean b) {
+                return b ? forTrue : forFalse;
+            } else {
+                return null;
+            }
+        }
+
+        @Override
+        public boolean containsKey(Object key) {
+            return key instanceof Boolean;
+        }
+
+        @Override
+        public boolean containsValue(Object value) {
+            return Objects.equals(value, forTrue) || Objects.equals(value, forFalse);
+        }
+
         @Override
         public Set<Map.Entry<Boolean, T>> entrySet() {
             return new AbstractSet<>() {