Report compile time error if const class has non-const initializer

Report compile time error if const class has non-const initializer

A recent language change requires classes with a const constructor
to have compile-time constant field initializer expressions. This
CL adds the check, and removes code to handle const instance fields,
which do not exist in Dart.

Fixes issue 18779.

R=regis@google.com

Committed: https://code.google.com/p/dart/source/detail?r=38910

[regis, 2014-08-05 21:24:07]

LGTM

[hausner, 2014-08-05 21:50:08]

Committed patchset #1 manually as r38910 (presubmit successful).