Avoid raceconditions in global activate, run and global run

[sigurdm]

Attempts to fix: #3165

This is done by:

global activate:

• Activating a global package into a temp-dir and renaming the dir into the right place.
  Two concurrent activations of the same package foo will not be able to both contribute to the resulting files, an observer will always see the result of one or the other or none in $PUB_CACHE/global_packages/foo.

• Creating binstubs in a temp-dir, making them executable and then moving them into the right place.
  It won't be possible to observe half a binstub, or for chmod to have the binstub removed before it can operate on it.

run/global run:

• Copying the incremental compilation data before compilation, and renaming the result to the right location.
  We already did a copy to preserve the incremental data, the change is to do the copy before compilation not after.

All of this builds on renaming being an atomic operation, another process will either see the old file or the new, never anything in-between.

There is still a possibility of confusing results if you concurrently dart pub global activate two different packages with overlapping binstub-names, then you can end up with any possible mixture of these binstubs - don't do that.

The pr contains quite some refactoring to allow global packages to exist in a temporary directory.

Also I took out support for pre dart-1.6 style lockfiles.
This fixes: #1111

[sigurdm]

@jakemac53 you might be interested in the changes to the precompilation (in dart.dart).

[jakemac53]

These changes LGTM