Remove cross-export warning

[srawlins]

There are a few examples of exporting (and even documenting) one package's elements from another package:

• package:flutter/widgets.dart exports package:characters/characters.dart
• package:test/expect.dart exports package:matcher/expect.dart
• package:test/fake.dart exports package:test_api/fake.dart

Theoretically these packages were getting warnings about cross-package exports, but the logic that determined if the warning should be shown was wrong: The logic was "If none of the re-exports are from the same package" (or, "if all of the re-exports are cross-package"). But that would even include the library where an element is declared, so I don't think the warning ever fired. Given that it wasn't working, and we don't need the warning (devs get use out of these cross-package exports), we should remove it.

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[srawlins]

I'll wait on this until characters is clean: dart-archive/characters#89

[srawlins]

CC @dart-lang/analyzer-team