Simplify dart2js StringBuffer constructor to be initializer-only.

Simplify dart2js StringBuffer constructor to be initializer-only.

The constructor is now small enough to always inline and the interpolation is usually constant-folded from the default argument, leading to the elimination of the silly looking store:

     t1 = new P.StringBuffer("");
     t1._contents = "";

-->
     t1 = new P.StringBuffer("");

R=floitsch@google.com

Committed: https://code.google.com/p/dart/source/detail?r=42212

[sra1, 2014-12-06 01:48:14]

sra@google.com changed reviewers:
+ floitsch@google.com

[sra1, 2014-12-09 04:05:45]

ping

[floitsch, 2014-12-09 10:05:59]

LGTM.

[sra1, 2014-12-09 17:51:43]

Committed patchset #2 (id:20001) manually as 42212 (presubmit successful).