Fix constant evaluation logic for String.length.

Fix constant evaluation logic for String.length.

Previously, we used static resolution to determine whether
String.length was being referred to, causing incorrect results in the
presence of bad type annotations.  Also, we were missing a null check,
causing constant evaluation to crash if an attempt was made to take
the length of a constant that couldn't be evaluated.

BUG=dartbug.com/23383
R=scheglov@google.com

Committed: https://code.google.com/p/dart/source/detail?r=45524

[Paul Berry, 2015-05-05 03:51:24]

paulberry@google.com changed reviewers:
+ scheglov@google.com

[scheglov, 2015-05-05 06:24:06]

LGTM

[Paul Berry, 2015-05-05 13:07:32]

Committed patchset #1 (id:1) manually as 45524 (presubmit successful).