Make pub run faster

[nex3]

"pub run" is currently very slow when running local executables. We should do what we can to mitigate this, including:

* Precompiling immutable transformers to snapshots on "pub get"

• Precompiling immutable dependencies to source
• Lazily load assets from untransformed dependencies
• Don't load transformers at all if we can prove that they won't be used
• Stop rewriting package imports (blocked on issue dart:isolate spawnUri should have a customizable package root #12474)
• Load files directly rather than over HTTP (blocked on issue Control how an isolate resolves imports #16368)

[munificent]

Marked this as blocking #20866.

[nex3]

Marked this as being blocked by #20866.

[nex3]

Unmarked this as blocking #20866.

[kasperl]

Added this to the 1.7 milestone.

[nex3]

r40392 caches immutable transformers, although they're not yet precompiled on "pub get".

[nex3]

Issue #20212 has been merged into this issue.

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cc @iposva-google.
cc @turnidge.

[nex3]

Removing Milestone-1.7. There's more performance improvements left to do, but "pub run" is about as fast as I was hoping for 1.7.

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Removed this from the 1.7 milestone.

[nex3]

Also removing issue #12474 as blocking, since it's now supported on the VM and that's all we need in pub.

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Unmarked this as being blocked by #12474.

[nex3]

With r41649 making the yaml package faster, "pub run" is now acceptably fast. Fixing issue #16366 and/or 20866 would still improve the speed substantially, though.

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Added Fixed label.