Fix release mode crash in String::DecodeURI.

Fix release mode crash in String::DecodeURI.

BUG=

Committed: https://code.google.com/p/dart/source/detail?r=31360

[Cutch, 2013-12-20 23:31:29]

TBR

[Cutch, 2013-12-20 23:32:17]

Committed patchset #2 manually as r31360 (presubmit successful).

[siva, 2013-12-21 00:10:36]

https://codereview.chromium.org/101653007/diff/30001/runtime/vm/object.cc
File runtime/vm/object.cc (right):

https://codereview.chromium.org/101653007/diff/30001/runtime/vm/object.cc#new...
runtime/vm/object.cc:14591: }
You seem to always break out of the while loop if an invalid character is seen
and then check and return original string on errors.

Why not just replace all breaks with "return str.raw()" and
get rid of the local variable 'valid'.

[turnidge, 2014-01-06 20:34:47]

https://codereview.chromium.org/101653007/diff/30001/runtime/vm/object.cc
File runtime/vm/object.cc (right):

https://codereview.chromium.org/101653007/diff/30001/runtime/vm/object.cc#new...
runtime/vm/object.cc:14591: }
On 2013/12/21 00:10:36, siva wrote:
> You seem to always break out of the while loop if an invalid character is seen
> and then check and return original string on errors.
> 
> Why not just replace all breaks with "return str.raw()" and
> get rid of the local variable 'valid'.

Agree with Siva that the code could be simpler.

I think the code is a bit more readable if we combine all of these tests
together:

  while (cpi.Next()) {
    if (IsPercent(cpi.Current())) {
      if (!cpi.Next() || !IsHexCharacter(cpi.Current()) ||
          !cpi.Next() || !IsHexCharacter(cpi.Current())) {
        return str.raw();
      }
      num_escapes += 2;
    } 
  }