Copy identifier names so that we don't hold on to the original source

Copy identifier names so that we don't hold on to the original source. If I'm reading the results right, this saves a significant amount of space.
Committed: https://code.google.com/p/dart/source/detail?r=7256

[scheglov, 2012-05-02 18:03:06]

Well, this may save memory, so LGTM.

But we don't know for sure without looking on numbers.
Would be good to have some internal action or part of perf test to show size of
the heap.
Only strongly reference objects.
Simplest solution for this in GWT was try to allocate huge array, catch OOME and
then print used memory.

And we could also use com.google.common.collect.Interners to save even more, but
again only under control of both CPU and heap performance.

[Brian Wilkerson, 2012-05-02 18:14:32]

> But we don't know for sure without looking on numbers.

Agreed. If I'm understanding the numbers provided by YourKit, doing the same
thing before and after the change resulted in a decrease (in strongly reachable
retained objects) from 179 MB to 155 MB. (Total memory was more significant,
from 209 MB to 158 MB, but that might be misleading.) It would be great to have
someone confirm those results, and as you said even better if such numbers were
consistently and accurately measured automatically.