Also introduces the PUB_ALLOW_PRERELEASE_SDK environment variable which can override this behavior. Valid values are `true` (default), `false`, and `quiet` (true but no log message about the override).

Fixes https://github.com/dart-lang/pub/issues/1692

R=jakemac@google.com

Review-Url: https://codereview.chromium.org//3012983003 .

This only applies to package dependencies, not the root package. This is to make it so the sdk constraint doesn't become a required field for all pubspecs in 2.x sdks.

A future update will make it so publishing a package requires an upper bound sdk constraint. 

* added overridable field 'featuresEnabled' for testing as well

Fixes https://github.com/dart-lang/sdk/issues/30246

As of https://github.com/dart-lang/sdk/commit/5b29ae48adcadc6f5f98c6ee937d4bb7c8414c83 the analyzer_cli no longer records summary deps, so the summary test started failing.

Fixes https://github.com/dart-lang/pub/issues/1687

fixes https://github.com/dart-lang/pub/issues/1684

Add "pub run --list" command to show all available executables #2

* removed "run --list" command
* buffer is used to output executables
* tests were adapted to verify --dev/--no-dev flag

This reverts commit 3e4890f1.

Does not fix https://github.com/dart-lang/pub/issues/1666
Tracking internally here http://b/63855519

Fixes https://github.com/dart-lang/pub-dartlang-dart/issues/230

* Fix deprecated uses from cli_util
* Move analysis_options to the correct name

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.