It turns out "is" is just a normal overridable operator. Who knew?

test/core/object/is.wren

@@ -0,0 +1,33 @@
+System.print(Num is Class) // expect: true
+System.print(true is Bool) // expect: true
+System.print(Fn.new { 1 } is Fn) // expect: true
+System.print(123 is Num) // expect: true
+System.print(null is Null) // expect: true
+System.print("s" is String) // expect: true
+
+System.print(Num is Bool) // expect: false
+System.print(null is Class) // expect: false
+System.print(true is Fn) // expect: false
+System.print(Fn.new { 1 } is Num) // expect: false
+System.print("s" is Null) // expect: false
+System.print(123 is String) // expect: false
+
+// Everything extends Object.
+System.print(Num is Object) // expect: true
+System.print(null is Object) // expect: true
+System.print(true is Object) // expect: true
+System.print(Fn.new { 1 } is Object) // expect: true
+System.print("s" is Object) // expect: true
+System.print(123 is Object) // expect: true
+
+// Classes extend Class.
+System.print(Num is Class) // expect: true
+System.print(null is Class) // expect: false
+System.print(true is Class) // expect: false
+System.print(Fn.new { 1 } is Class) // expect: false
+System.print("s" is Class) // expect: false
+System.print(123 is Class) // expect: false
+
+// Ignore newline after "is".
+System.print(123 is
+  Num) // expect: true

test/core/object/nonclass_on_right.wren

@@ -0,0 +1 @@
+1 is false // expect runtime error: Right operand must be a class.