Oh look, I figured out how to emit `istrue`, so that's tested, too.

core/tests/regression_tests.rs

@@ -274,6 +274,7 @@ swf_tests! {
     (as3_istype, "avm2/istype", 1),
     (as3_instanceof, "avm2/instanceof", 1),
     (as3_truthiness, "avm2/truthiness", 1),
+    (as3_falsiness, "avm2/falsiness", 1),
 }
 
 // TODO: These tests have some inaccuracies currently, so we use approx_eq to test that numeric values are close enough.

core/tests/swfs/avm2/falsiness/Test.as

@@ -0,0 +1,57 @@
+package {
+	public class Test {
+	}
+}
+
+function assert_falsiness(val) {
+	if (!val) {
+		trace("Value is falsy");
+	} else {
+		trace("Value is truthy");
+	}
+}
+
+trace("//if (!true)");
+assert_falsiness(true);
+
+trace("//if (!false)");
+assert_falsiness(false);
+
+trace("//if (!null)");
+assert_falsiness(null);
+
+trace("//if (!undefined)");
+assert_falsiness(undefined);
+
+trace("//if (!\"\")");
+assert_falsiness("");
+
+trace("//if (!\"str\")");
+assert_falsiness("str");
+
+trace("//if (!\"true\")");
+assert_falsiness("true");
+
+trace("//if (!\"false\")");
+assert_falsiness("false");
+
+trace("//if (!0.0)");
+assert_falsiness(0.0);
+
+trace("//if (!NaN)");
+assert_falsiness(NaN);
+
+trace("//if (!-0.0)");
+assert_falsiness(-0.0);
+
+trace("//if (!Infinity)");
+assert_falsiness(Infinity);
+
+trace("//if (!1.0)");
+assert_falsiness(1.0);
+
+trace("//if (!-1.0)");
+assert_falsiness(-1.0);
+
+trace("//if (!new Object())");
+assert_falsiness({});

core/tests/swfs/avm2/falsiness/output.txt

@@ -0,0 +1,30 @@
+//if (!true)
+Value is truthy
+//if (!false)
+Value is falsy
+//if (!null)
+Value is falsy
+//if (!undefined)
+Value is falsy
+//if (!"")
+Value is falsy
+//if (!"str")
+Value is truthy
+//if (!"true")
+Value is truthy
+//if (!"false")
+Value is truthy
+//if (!0.0)
+Value is falsy
+//if (!NaN)
+Value is falsy
+//if (!-0.0)
+Value is falsy
+//if (!Infinity)
+Value is truthy
+//if (!1.0)
+Value is truthy
+//if (!-1.0)
+Value is truthy
+//if (!new Object())
+Value is truthy