Fix dartc in optimized mode (broken in r3358).

Fix dartc in optimized mode (broken in r3358).

Add checks that ensure that keys used for array access are integers.

Fix bug in the optimization strategy that ended up generating the wrong
code for "native" classes.

R=floitsch@google.com,mmendez@google.com
BUG=
TEST=

Committed: https://code.google.com/p/dart/source/detail?r=3390

[floitsch, 2012-01-18 12:34:42]

LGTM!

[mmendez, 2012-01-18 16:27:54]

lgtm

The inconsistency between the class modifiers and the native class name was the
root cause behind the failure of
http://code.google.com/p/dart/source/detail?r=3356.  Thanks for fixing it in
this patch.

https://chromiumcodereview.appspot.com/9250005/diff/3001/compiler/java/com/go...
File
compiler/java/com/google/dart/compiler/backend/js/BasicOptimizationStrategy.java
(right):

https://chromiumcodereview.appspot.com/9250005/diff/3001/compiler/java/com/go...
compiler/java/com/google/dart/compiler/backend/js/BasicOptimizationStrategy.java:426:
if (classElement.getNativeName() != null) {
Nit: I'm not sure how many other places might assume that native classes have
the native bit set in the modifiers.  The fact that the modifier does not jive
with the native name attribute is confusing.

Another option is to leave this as classElement.getModifiers and append
"modifiers = modifiers.makeNative();" to DartParser.parseClass line 611.