Use Compiler API in Loader.

Now our command line always runs through our Compiler API.

Fixes #1324
```
feature/ArrowFunctions/Skip_Frozen.js
feature/ArrowFunctions/Skip_InitializerShorthand.js
feature/ArrowFunctions/Skip_SoftBind.js
feature/Classes/Skip_DerivedButton.js
feature/Classes/Skip_HTMLBlockquoteElement.js
feature/GeneratorComprehension/Skip_Closure.js
feature/Syntax/Empty.js
```
or any API call that gets an empty string for source.  We could make it an error but typical compilers I've used do not.