Ignore uri_does_not_exist in analyzer/DDC

[natebosch]

• Drop strong-mode option - it is the default.
• Add ignore for uri_does_not_exist.

Since imports are crawled while finding modules we have separate error
reporting already for missing imports. We don't need DDC to find these
for us.

[natebosch]

Here's what the error looks like today:

[SEVERE] build_web_compilers|entrypoint on web/main.dart:
Unable to find modules for some sources, this is usually the result of either a
bad import, a missing dependency in a package (or possibly a dev_dependency
needs to move to a real dependency), or a build failure (if importing a
generated file).

Please check the following imports:

`import 'bad.dart';` from dart_repro|web/main.dart at 1:1

[jakemac53]

I think this is reasonable, can you also remove the comments here just to confirm all is good https://github.com/dart-lang/build/blob/master/_test/test/config_specific_import_test.dart#L7

[natebosch]

Ugh, DDC seems to ignore this. I don't know if it ignores --options altogether, or only ignores if you try to change error levels.

In any case I don't think we can hack around it on our end.

[natebosch]

On the plus side, the original request from dart-lang/sdk#29713 is now essentially granted. On the downside we can't hack around dart-lang/sdk#34177 using this strategy.