Generate and consume the package spec (.packages)

[sethladd]

This is the main tracking bug to implement the package spec proposal. This proposal has been accepted (but has not yet gone through TC52), and work has begun on implementing it.

This is the main tracking bug. Each platform's specific bug is below:

• VM: VM: Implementing the package spec proposal #23369
• VM: --packages doesn't deadlock: VM deadlocks when passed the "--packages" flag #23991
• dart2js: dart2js: Implementing the package spec proposal #23371, first released in 1.12.0-dev.0.0
• Analyzer: Analyzer: implementing package spec proposal (.packages) #23373
• Analysis Server: Analysis Server: implement support for .packages #23925
• Pub: global activate: write a .packages when globally installing a package pub#22
• Pub: merge new version into sdk: Merge the latest pub to dev #24094
• Pub: Pub: Implementing package spec proposal pub#16, first released in 1.12.0-dev.0.0
• Fix self-references in Pub: Self-references in generated .packages files point to the pubcache. pub#1294, will be released in 1.12.0-dev.3.0
• WebStorm: https://youtrack.jetbrains.com/issue/WEB-16466
• Fetching a .packages file after an http: entry point: Request: requesting a .packages file from http: import of an entry point #23952
• Test package_config on the packagebots Request: testing package_config on the packagebots #24092

Note that additional work will be needed in phase 2, see #24112

[sethladd]

Marked this as being blocked by #23373.

[sethladd]

Marked this as being blocked by #23374.

[sethladd]

Added Accepted label.

[sethladd]

See also this JetBrains plugin tracking issue: https://youtrack.jetbrains.com/issue/WEB-16466

[sethladd]

Added this to the 1.11 milestone.
Added Priority-High label.

[sethladd]

And we're accepted! dart-archive/dart_enhancement_proposals#5

[sethladd]

Added Started label.

[sigmundch]

Marked this as blocking #23565.