Check that RHS of "is" is a class.

test/is/is.wren

@@ -0,0 +1,32 @@
+IO.print(Num is Class) // expect: true
+IO.print(true is Bool) // expect: true
+IO.print((fn 1) is Function) // expect: true
+IO.print(123 is Num) // expect: true
+IO.print(null is Null) // expect: true
+IO.print("s" is String) // expect: true
+
+IO.print(Num is Bool) // expect: false
+IO.print(null is Class) // expect: false
+IO.print(true is Function) // expect: false
+IO.print((fn 1) is Num) // expect: false
+IO.print("s" is Null) // expect: false
+IO.print(123 is String) // expect: false
+
+// Everything extends Object.
+IO.print(Num is Object) // expect: true
+IO.print(null is Object) // expect: true
+IO.print(true is Object) // expect: true
+IO.print((fn 1) is Object) // expect: true
+IO.print("s" is Object) // expect: true
+IO.print(123 is Object) // expect: true
+
+// Classes extend Class.
+IO.print(Num is Class) // expect: true
+IO.print(null is Class) // expect: false
+IO.print(true is Class) // expect: false
+IO.print((fn 1) is Class) // expect: false
+IO.print("s" is Class) // expect: false
+IO.print(123 is Class) // expect: false
+
+// TODO: Non-class on RHS.
+// TODO: Precedence and associativity.

test/is/nonclass_on_right.wren

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