Assert that ImportDeclaration and ExportDeclaration match only the
ModuleItem symbol.

According to the definition of HostResolveImportedModule, it is
acceptable for an implementation to throw a SyntaxError in the event
that a requested module can neither be found nor created:

> If a Module Record corresponding to the pair referencingModule,
> specifier does not exist or cannot be created, an exception must be
> thrown.

In order to reliably detect a SyntaxError in response to the correct
interpretation of the grammar (and not a SyntaxError from an *incorrect*
interpretation of the grammar followed by a failure to resolve the
requested module), the ModuleSpecifier of ExportDeclarations should
describe a valid resource.