Incorrect assertion for positional params on keyword usage when keyword is not in use

[Chris Thoburn]

Given any keyword (such as the example below of modifier) the following assertion is triggered during the build when the keyword is encountered in a non-keyword template position as an argument.

 (modifier) requires a modifier definition or identifier as its first positional parameter, did not receive any parameters.: 

|
|  modifier
|

Code examples which trigger this assertion incorrectly include usage of both this and @ scoping, for instance:

<MyComponent @foo={{@modifier}} /> <!-- errors on @modifier -->
<OtherComponent @foo={{this.modifier}} /> <!-- errors on this.modifier -->