Pub should parse Dart code to do more validation before uploading

[nex3]

Pub should do various sorts of validation that requires Dart code to be parsed. In particular:

Errors:

* Part files in "lib" outside of "lib/src".

• Non-"package:" imports in "bin".
• Examples importing from "lib/src".
• Dart code with relative imports to files that don't exist.
• Dart code with "package:" imports to files in this package that don't exist.
• Dart code with "package:" imports for packages that aren't in the pubspec.

Warnings:

* Dart files in "bin" without main() methods.

• Non-"package:" imports in "test" or "example".
• Imports in "test" or "example" that reach outside the package.

[munificent]

Added this to the Later milestone.

[munificent]

Removed Type-Defect label.
Added Type-Enhancement label.

[kasperl]

Removed this from the Later milestone.
Added Oldschool-Milestone-Later label.

[kasperl]

Removed Oldschool-Milestone-Later label.

[DartBot]

This issue has been moved to dart-lang/pub#343.