Allow externalization of canonical strings also. Only restrict canonical strings

Allow externalization of canonical strings also. Only restrict canonical strings
in the VM isolate heap.
Committed: https://code.google.com/p/dart/source/detail?r=17437

[Ivan Posva, 2013-01-17 07:19:39]

Please address the two byte strings as well in this change.

Thanks,
-Ivan

https://chromiumcodereview.appspot.com/11969029/diff/1/vm/object.cc
File vm/object.cc (right):

https://chromiumcodereview.appspot.com/11969029/diff/1/vm/object.cc#newcode2268
vm/object.cc:2268: ASSERT(name.IsOneByteString() ||
name.IsExternalOneByteString());
I do not understand why we are limiting this to one byte strings here. There is
no code here that expects this symbol to be a one byte string.

Here and many other places.

I am pretty certain that we can end up in these functions with two byte strings
when coming in through the C API.

[siva, 2013-01-17 19:03:32]

Removed OneByteString assertions in the lookup functions and beefed up
EqualsIgnorePrivateKey to allow for TwoByteStrings.

PTAL

[Ivan Posva, 2013-01-23 03:31:46]

LGTM -ip