Support const modifier in fields, variables

Support const modifier in fields, variables

Issue 3551

This change adds const as a valid keyword to declare fields and
variables. The mandatory initializer has to be a compile time
const expression.

Static final variables still have to be initialized with
compile time constant expressions. I will add non-const static
finals (issue 3558) in another change.

Other small changes:
- allow null in some compile time constant expressions.
- allow only numbers, booleans, strings and null in compile time
  constant string interpolation.
Committed: https://code.google.com/p/dart/source/detail?r=9992

[hausner, 2012-07-27 18:20:43]

Constantly making Dart better :)

[siva, 2012-07-27 20:32:37]

lgtm