Fix comma splice in on modifier error message.

[muziejus]

No description provided.

[rwjblue]

Nice thank you!!!

[rwjblue]

Looks like we have a few tests checking this error message that will need to be updated also:

not ok 1453 Chrome 94.0 - [4 ms] - [integration] jit :: {{on}} Modifier: asserts when callback is undefined
1527
    ---
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        actual: >
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            Error: You must pass a function as the second argument to the `on` modifier; you passed undefined. While rendering:
1530
            
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            this.foo
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        expected: >
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            /You must pass a function as the second argument to the `on` modifier, you passed undefined. While rendering:\n\nthis.foo/
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        stack: >
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                at OnTest.assertsWhenCallbackIsUndefined [as asserts when callback is undefined] (http://localhost:7357/148153064161/tests/assets/tests.js:14317:25)
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                at Object.<anonymous> (http://localhost:7357/148153064161/tests/assets/glimmer-vm.js:18255:35)
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                at runTest (http://localhost:7357/148153064161/tests/assets/qunit.js:3048:30)
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                at Test.run (http://localhost:7357/148153064161/tests/assets/qunit.js:3034:6)
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                at http://localhost:7357/148153064161/tests/assets/qunit.js:3265:12
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                at processTaskQueue (http://localhost:7357/148153064161/tests/assets/qunit.js:2621:24)
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        negative: >
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            false
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        browser log: |

[rwjblue]

Those tests are here

glimmer-vm/packages/@glimmer/integration-tests/test/modifiers/on-test.ts

Lines 372 to 391 in 4f1bef0

	@test
	'asserts when callback is missing'(assert: Assert) {
	assert.throws(() => {
	this.render(`<button {{on 'click'}}>Click Me</button>`);
	}, /You must pass a function as the second argument to the `on` modifier/);
	}
	@test
	'asserts when callback is undefined'(assert: Assert) {
	assert.throws(() => {
	this.render(`<button {{on 'click' this.foo}}>Click Me</button>`);
	}, /You must pass a function as the second argument to the `on` modifier, you passed undefined. While rendering:\n\nthis.foo/);
	}
	@test
	'asserts when callback is null'(assert: Assert) {
	assert.throws(() => {
	this.render(`<button {{on 'click' this.foo}}>Click Me</button>`, { foo: null });
	}, /You must pass a function as the second argument to the `on` modifier, you passed null. While rendering:\n\nthis.foo/);
	}

[muziejus]

Thanks for your patience. I changed the tests, too.

[locks]

Thanks again :)