Previously we were always loading an AssetEnvironment when we
precompiled any executable. Now we only load that environment if some
package dependended on by some executable actually uses a transformer.
Otherwise, we precompile from disk. The new approach is substantially
faster and produces better error messages.

Closes #1473

Where /var/ is really /private/var/

Test package APIs rather than resource APIs.

Closes #1446

The actual change itself is tiny. However, almost all of pub's existing
integration tests validated the correct output by looking at the
contents of the generated "packages" dirs. :(

I fixed most of those to instead validate against the .packages file.

I didn't change the Git ones because the .packages files end up
containing paths that point to specific Git commits. Getting that
working felt like more work than it was worth right now.

When we fully remove support for packages directories, we'll want to
fix that.

Fix #1450.

R=nweiz@google.com

Review URL: https://codereview.chromium.org//2377063002 .

Closes #1118
Closes #1448

Rather than resetting the entire line each time with \r, just erase the
time.

BUG=https://github.com/dart-lang/pub/issues/1446
R=rnystrom@google.com

Review URL: https://codereview.chromium.org/2357263002 .

This allows packages that override only dev dependencies to be
published.

Closes #1444

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//2276143002 .

When a dependency is overridden, any dependency it has back on the root
package is now ignored. This makes it easier for users to develop one
portion of a circular dependency.

Closes #1443

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//2276943003 .

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//2256623003 .

This replaces the hidden --no-package-symlinks flag.

Closes #1340

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//2250643003 .

Fixes #1438

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//2233893004 .

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//2199183002 .

We had started using newer APIs without updating the dependencies
accordingly.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//2198993003 .

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//2184303002 .

Closes #1430

R=floitsch@google.com, kevmoo@google.com

Review URL: https://codereview.chromium.org//2148413002 .

Currently Flutter is the only supported SDK.

Closes #1431

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//2174913002 .

Packages with Flutter constraints should only be published with SDK
constraints that restrict the SDK to versions that support Flutter
constraints.

Closes #1432

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//2172883002 .

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.