Fix crashes in scanner.

Fix crashes in scanner.

R=johnniwinther@google.com

Committed: https://code.google.com/p/dart/source/detail?r=30782

[ahe, 2013-11-26 08:51:15]

The cause of the crashes were a cast to BeginGroupToken that wasn't valid and an
attempt to read after EOF, in particular after seeing bad string interpolation
like '''$'''.

In addition to fixing the crashes, I also improved the error message for said
string interpolation. Before it would be diagnosed as an unterminated {, now the
scanner avoid consuming illegal characters after $ in string interpolation.

[Johnni Winther, 2013-11-26 12:11:11]

lgtm

https://codereview.chromium.org/87813002/diff/1/dart/sdk/lib/_internal/compil...
File dart/sdk/lib/_internal/compiler/implementation/scanner/parser.dart (right):

https://codereview.chromium.org/87813002/diff/1/dart/sdk/lib/_internal/compil...
dart/sdk/lib/_internal/compiler/implementation/scanner/parser.dart:1782: token =
expect('(', token);
Add comment that [expect] ensures that `begin.endGroup` is only accessed when
[token] is a [BeginGroupToken].

https://codereview.chromium.org/87813002/diff/1/dart/sdk/lib/_internal/compil...
File dart/sdk/lib/_internal/compiler/implementation/scanner/scanner.dart
(right):

https://codereview.chromium.org/87813002/diff/1/dart/sdk/lib/_internal/compil...
dart/sdk/lib/_internal/compiler/implementation/scanner/scanner.dart:203:
error('blah');
Strange error message!

[ahe, 2013-11-29 10:48:38]

Thank you, Johnni!

https://codereview.chromium.org/87813002/diff/1/dart/sdk/lib/_internal/compil...
File dart/sdk/lib/_internal/compiler/implementation/scanner/parser.dart (right):

https://codereview.chromium.org/87813002/diff/1/dart/sdk/lib/_internal/compil...
dart/sdk/lib/_internal/compiler/implementation/scanner/parser.dart:1782: token =
expect('(', token);
On 2013/11/26 12:11:12, Johnni Winther wrote:
> Add comment that [expect] ensures that `begin.endGroup` is only accessed when
> [token] is a [BeginGroupToken].

Done.

https://codereview.chromium.org/87813002/diff/1/dart/sdk/lib/_internal/compil...
File dart/sdk/lib/_internal/compiler/implementation/scanner/scanner.dart
(right):

https://codereview.chromium.org/87813002/diff/1/dart/sdk/lib/_internal/compil...
dart/sdk/lib/_internal/compiler/implementation/scanner/scanner.dart:203:
error('blah');
On 2013/11/26 12:11:12, Johnni Winther wrote:
> Strange error message!

Done.

[ahe, 2013-11-29 12:50:21]

Committed patchset #4 manually as r30782 (presubmit successful).