Re-introduce string concatenation

Re-introduce string concatenation

This change re-introduces concatenation of adjacent
string literals, i.e. the compiler concatenates "abc" "123"
into the literal "abc123" at compile time. If the literals
contain interpolations, the concatenation happens at
runtime.

The change also re-introduces interpolated compile-time
string literal constants, but the feature is disabled because
several tests need to be changed.
Committed: https://code.google.com/p/dart/source/detail?r=4991

[hausner, 2012-03-03 02:01:16]

A number of tests need to be fixed or deleted before we can turn on the
compile-time const string interpolation.

[Ivan Posva, 2012-03-05 22:57:12]

LGTM -ip