This change adds code that detects if an expression can definitively
never be constant. We use this to analyze the initializer expressions
in const constructors. This check is not entirely water tight, but
together with the checks in canonicalization code catches most
illegal initializer expressions.
The const constructor of class Symbol turns out to be illegal. The
name verification check can't be part of the constructor code.
R=iposva@google.com
Review URL: https://codereview.chromium.org//18649003
git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@24749 260f80e4-7a28-3924-810f-c04153c831b5
hausner@google.com