Fix bug in optimizing string .length loads.

Fix bug in optimizing string .length loads.

We're not allowed to hoist a .length load if the corresponding
class-check is not also hoisted out of a loop.

TEST=tests/language/optimized_string_charcodeat_test.dart
Committed: https://code.google.com/p/dart/source/detail?r=18519

[Vyacheslav Egorov (Google), 2013-02-14 15:14:57]

LGTM

But can we just forbid externalization to change actual length? It does not make
any sense to me.

Alternatively we should introduce dependencies.

[Florian Schneider, 2013-02-14 15:46:11]

On 2013/02/14 15:14:57, Vyacheslav Egorov (Google) wrote:
> LGTM
> 
> But can we just forbid externalization to change actual length? It does not
make
> any sense to me.
> 
> Alternatively we should introduce dependencies.

Here the problem is not changing the length, but hoisting the inlined load above
the check-class. I think explicit dependency between the check-class and the
load would be the best. Let's talk about that off-line.

[Vyacheslav Egorov (Google), 2013-02-14 16:00:29]

Yes, you are absolutely right. 

You can however still hoist if def->Type() is string.

But we should really introduce dependencies to make things more robust.