simplify dart 2.0 command line execution

[a-siva]

Currently we have numerous options that need to be set when invoking dart 2.0 on the command line.

1. Need to specify platform binaries location
2. Need to specify location of the front end snapshot

Would be more ideal if we

• generated the front end snapshot as part of the sdk
• linked in the platform binaries into the executable
• auto located the front end snapshot from the sdk

Would also be ideal if we could generate a core snapshot of platform binaries and linked it
into the executable instead of the platform binaries. This has the issue of figuring out how
we provide the platform binaries to the front end which requires it for incremental compilations.
(the snapshot has the kernel representation of all the core libraries, maybe the interface to the
FE can be changed to allow dart to specify the platform binaries from the snapshot).

[a-siva]

We have gotten rid of the requirement to specify the following options in order to run in 2.0 mode, we are down to being able to do 'dart --preview_dart_2 myapp.dart' to be able to run in 2.0 mode.