Fix to workaround non-propagation of exception stack-traces by dartc.

Fix to workaround non-propagation of exception stack-traces by dartc.

Net-net: The previous work-around, assigning '' to the trace var inside the try block, had stopped working.  Somewhere along the line dartc started properly respecting try/catch block scoping of vars.

In an attempt to verify that this fix hasn't broken anything unexpected, I ran test.py and confirmed that all tests still pased and re-ran it with a broken test and confirmed that a stack trace was emitted.

Here's how I built and ran:

./tools/build.py --arch=ia32
./tools/test.py --mode=release --component=dartium client -v --keep_temporary_files
Committed: https://code.google.com/p/dart/source/detail?r=1639

[Siggi Cherem (dart-lang), 2011-11-17 22:03:59]

lgtm, but let's try some additional tests as follows:
./tools/build.py --arch=ia32
./tools/test.py --component=chromium

Basically I changed 'dartium' for 'chromium' (the former doesn't invoke dartc),
I didn't put 'release' because build.py is probably compiling the debug version,
and I didn't include 'client' so this covers language and corelib tests too.

http://codereview.chromium.org/8586041/diff/1/client/testing/unittest/shared....
File client/testing/unittest/shared.dart (right):

http://codereview.chromium.org/8586041/diff/1/client/testing/unittest/shared....
client/testing/unittest/shared.dart:175: //TODO remove guard once dartc reliably
propagates traces
TODO -> TODO(pquitslund) or TODO(sigmund)

[pquitslund, 2011-11-17 23:34:23]

On 2011/11/17 22:03:59, sigmund wrote:
> lgtm, but let's try some additional tests as follows:
> ./tools/build.py --arch=ia32
> ./tools/test.py --component=chromium

As per our conversation, I did this, ultimately falling back to running
sequentially to sidestep known issues batching dartc tests on the mac jvm:

./tools/test.py -j1 --component=chromium

Run this way, all tests pass.



> 
> Basically I changed 'dartium' for 'chromium' (the former doesn't invoke
dartc),
> I didn't put 'release' because build.py is probably compiling the debug
version,
> and I didn't include 'client' so this covers language and corelib tests too.
> 
>
http://codereview.chromium.org/8586041/diff/1/client/testing/unittest/shared....
> File client/testing/unittest/shared.dart (right):
> 
>
http://codereview.chromium.org/8586041/diff/1/client/testing/unittest/shared....
> client/testing/unittest/shared.dart:175: //TODO remove guard once dartc
reliably
> propagates traces
> TODO -> TODO(pquitslund) or TODO(sigmund)

Done.