Fix crashing bug due to the fact that external one byte strings cannot be escaped

Fix for issue where external one byte strings cannot be escaped. This occured in practice when requesting all library names for a Polymer.dart example The likely reason why the issue occurs in this case but not others is that some of the libraries are generated using data urls. Added some tests for EscapeSpecialCharacters as it did not have any tests.

BUG=
R=hausner@google.com

Committed: https://code.google.com/p/dart/source/detail?r=28073

[Jacob, 2013-09-30 21:07:57]

I'm not sure who the right person on the VM team is to review this. I am sending
to hausner as the bug was exposed by a debugger API method to get library names.
Please reassign as appropriate.
I'm not sure whether the right solution is to directly implement the method as I
did for ExternalOneByteString
or to just duplicate the String as an internal string as I did for
ExternalTwoByteString.  ExternalOneByteString is the only case I care about in
Dartium so I implemented it natively (basically via copy and paste from the
OneByteString code).

[hausner, 2013-09-30 22:54:37]

LGTM with comments

https://codereview.chromium.org/25344002/diff/1/runtime/vm/object.cc
File runtime/vm/object.cc (right):

https://codereview.chromium.org/25344002/diff/1/runtime/vm/object.cc#newcode1...
runtime/vm/object.cc:13691: intptr_t index = 0;
declare index just before the loop below where it's used?

https://codereview.chromium.org/25344002/diff/1/runtime/vm/object.cc#newcode1...
runtime/vm/object.cc:13712: return OneByteString::null();
It it correct to return null when the input string is of length 0?

[Jacob, 2013-09-30 23:53:27]

https://codereview.chromium.org/25344002/diff/1/runtime/vm/object.cc
File runtime/vm/object.cc (right):

https://codereview.chromium.org/25344002/diff/1/runtime/vm/object.cc#newcode1...
runtime/vm/object.cc:13691: intptr_t index = 0;
On 2013/09/30 22:54:37, hausner wrote:
> declare index just before the loop below where it's used?

Fixed that here and in the code this method was copied from modulo a couple
tweaks.

https://codereview.chromium.org/25344002/diff/1/runtime/vm/object.cc#newcode1...
runtime/vm/object.cc:13712: return OneByteString::null();
On 2013/09/30 22:54:37, hausner wrote:
> It it correct to return null when the input string is of length 0?

That is what the existing code did but I agree it seems bad.  Added a test for
that case as well and fixed this code and the other corresponding code. 
Previously it was the case that 
escaping "" resulted in null which is definitely not what you want.

[Jacob, 2013-10-01 00:08:08]

Committed patchset #2 manually as r28073 (presubmit successful).