Pub build filters out .dart files in lib/, even in debug mode

[munificent]

In debug mode, Dart files should not be filtered out by pub build. Currently they are if they live in a package's lib directory, regardless of build mode.

See this thread:

https://groups.google.com/a/dartlang.org/forum/#!topic/misc/JjilMA9pQXE

[munificent]

We should do this, but note that it does not completely solve Dart files in the build output.

If there are Dart files with "package:" imports in subdirectories of the build directory, they won't work. See: https://code.google.com/p/dart/issues/detail?id=16649

[munificent]

Marked this as blocking #16649.

[munificent]

This will be implicitly fixed by: https://codereview.chromium.org/135153012/

But I'm leaving this bug open to track writing a test that .dart files specifically are copied over.

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Added Started label.

[munificent]

https://codereview.chromium.org//149213009

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Added Fixed label.

[DartBot]

This issue has been moved to dart-lang/pub#846.