Separated the concept of apparent types from contextual types for string literal types.
Separated the concept of apparent types from contextual types for string literal types.

In most cases, expressions are interested in the apparent type of the
contextual type. For instance:

    var x = { hasOwnProperty(prop) { /* ... */ };

In the above, 'prop' should be contextually typed as 'string' from the
signature of 'hasOwnProperty' in the global 'Object' type.

However, in the case of string literal types, we don't want to get the
apparent type after fetching the contextual type. This is because the
apparent type of the '"onload"' string literal type is the global 'String'
type. This has adverse effects in simple assignments like the following:

    let x: "onload" = "onload";

In this example, the right-hand side of the assignment will grab the type
of 'x'. After figuring out the type is "onload", we then get the apparent
type which is 'String'. This is problematic because when we then check the
assignment itself, 'String's are not assignable to '"onload"'s.

So in this case, we grab the contextual type *without* getting its
apparent type.

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