bug(transpiler): Support optional arguments in annotations.

Clean-up: move annotaitons out of fixtures/annotations, since we have
the @CONST annotation in the transpiler already.
feat(transpiler): Transform template strings to triple quoted Dart strings
docs(di): simple typo
fix(lang): fix small typo.
fix(transpiler/dart): re-exporting only some bindings

```
export {Foo, Bar} from ‘./foo’;
==>
export ‘./foo’ show Foo, Bar;
```
feat(transiler/dart): re-export imported vars

```
import {Foo} from ‘./foo’;
var localVar = true;
export {Foo, localVar};

===>

import ‘./foo’ show Foo;
export ‘./foo’ show Foo;
var localVar = true;
```

Closes #41