We still need to verify that a Flutter SDK constraint is only
used with an appropriate Dart SDK constraint.

See #1432

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//2165423002 .

This brings these tests in line with others, making them integration
tests that invoke a pub process rather than testing the version solver
APIs directly. This will make it easier to add Flutter support in the
future (see #1431 and #1432).

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//2172523002 .

Change duration formatting in niceDuration

This allows PackageName equality to include description equality, and it
substantially cleans up other logic dealing with sources.

See #912

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//2079303003 .

This splits Source into two classes, Source and LiveSource. The former
is completely independent of the system cache directory, and mostly
contains source metadata and logic about manipulating descriptions.

SourceRegistry now deals strictly in Sources, with LiveSources being
handled by SystemCache. This means that classes like Pubspec and
LockFile no longer have transitive references to the system cache
directory.

This will also make it possible to clean up PackageName equality logic
by making PackageNames track their concrete Sources rather than just
their source names.

See #912

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//2044253003 .

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//2046563003 .

This reverts back to 20ddf7ce.

The intermediate commits were meant to go to a development branch.