Expression extractor does not find templatesUrls in 'web' directory

[rkirov]

Sample app:
web/main.dart

@Component(
  templateUrl: 'myTemplate.html'

myTemplate.html is in web/ but the transformer looks for it in package root.

pub build
> line 11, column 1 of web/main.dart: Unable to find mytmp.html at shadow_demo|mytmp.html

This manifests rather nastily during runtime as a Missing getter: (o) => o.foo error.

[rkirov]

Current workarounds:

• move components and their templates into lib/ (best practice for large applications) and use packages/<my_package>/myTemplate.html.
• specify html_files in the angular transformer as follows:

transformers:
- angular:
    html_files:
      web/mytmp.html

[naomiblack]

@rkirov can we move this over to the wiki instead of an issue? :)

[rkirov]

Will be closed by #1652.