Changes for broken struct issue with text parser

[desaikd]

Issue #682

Description of changes:
Changes for throwing error on broken struct as input for text parser.

Input:

{broken

Output before change:

Struct { _fields: [Object: null prototype] {} }  // null struct

Output after change:

Error: expected ':'  // throws errror

Test:
added unit tests for text reader and dom load

[zslayton]

Good find! Could you add a unit test for this?

[asemqaffaf]

why wouldn't you use const instead of let since you don't modify or change the value of invalidIonToRead and ionReader

[zslayton]

Upgrading let to const where appropriate is something we intend to do project-wide via one of our linting tools but haven't had the opportunity yet. Once we do, new code will be required to const where possible.