Closes #1593

Packages that don't declare an SDK constraint will now be considered
compatible with SDK versions matching <2.0.0-dev.infinity.

Closes #1657

Partially addresses #1593

Enabling multiple dependencies is necessary for some use-cases for
issue #1593 like having a package expose a feature that enables a
feature in a dependency, or having a package expose a "testing"
feature and also depend on test from its dev dependencies.

For consistency with features, this also removes the restriction that
a pubspec may not have a normal and dev dependency on the same
package.

Support features with "pub global activate".

Partially addresses #1593

Partially addresses #1593

This allows packages to declare features with dependencies, and to opt
in to those features in dependency declarations.

See #1593

Wait for --build-delay milliseconds between filesystem events before scheduling new builds.

Add support for Git dependencies on subdirectories

This isn't going to make it into -dev.4.0.

This will make it easier to add features to the pubspec and verify that
they're only published with appropriate SDK constraints.

It also simplifies the dependency validator logic around caret
constraints, since it's a lot less likely that anyone will want to
support an SDK older than 1.8.0 at this point.

We used to name these after the package itself, but that makes it
difficult to share repositories between sub-packages.

Refactor PackageName.

This more clearly indicates the type of data the class contains. We
were already using in more places than just dependencies, so it makes
sense to have a name that reflects that.

These classes are largely independent of the Package class.

We were already passing allowTrailingOptions to all ArgParser
call-sites, so this doesn't require any code changes.

Remove unsupported --unsafe-angular2-whitelist parameter.

BUG=
R=jakemac@google.com, vsm@google.com

Review-Url: https://codereview.chromium.org//2939453002 .

Add Travis support.

4 is just a guess here.

This helps ensure that tests behave the same in environments (such as
Travis) where PUB_ENVIRONMENT is already set.