Allow mixing of deferred/non components

[matanlurey]

Ran into this while debugging #1539.

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If you use @deferred in a certain way (details pending), the following is emitted:

import '1539_nested_template_test.dart' deferred as deflib0;

class ViewNestedTemplateTest0 extends AppView<deflib0.NestedTemplateTest> { ... }

That is illegal in Dart, resulting in:

[error] The deferred type 'deflib0.NestedTemplateTest' can't be used in a declaration, cast or type test

This is based, I believe, on trying to re-use existing import statements. We should not be treating deferred libraries as something usable for types. This might be trivial to fix, it might not, but it makes the usability of @deferred fairly brittle :(

[matanlurey]

Source of issue is here, might be trivial actually:

https://github.com/dart-lang/angular/blob/5ad73932b7885ce78b1371bbe79628f5fec3d6d6/angular/lib/src/compiler/output/dart_emitter.dart#L576-L617

[matanlurey]

After talking offline, we'll likely just fail instead of waiting for DDC/Dart2JS to fail.

[matanlurey]

... so turns out, this actually was just sort of a bug in dart_emitter.dart.

It was fixable 🙌 ... but required refactoring dart_emitter.dart a tiny bit. Not a huge deal.