Updates incremental compilation tests to work w/o code generation

Updates incremental compilation tests to work w/o code generation

Removes end2end tests that presume code generation

Committed: https://code.google.com/p/dart/source/detail?r=4592

[zundel, 2012-02-25 21:08:01]

This fix was on my list of "TODOs" before removing the backend.  The tests I
removed and updated in this patch 
required the code generating backend.  To fix incremental compilation, I added a
timestamp file for each dart unit 
that gets updated if it is analyzed.  in the editor, we might want to use some
sort of 'memory only' artifact provider that would just hold these timestamps,
or swap out the ArtifactProvider construct with something new all together.

[scheglov, 2012-02-25 21:47:17]

LGTM

http://codereview.chromium.org/9464044/diff/1/compiler/javatests/com/google/d...
File compiler/javatests/com/google/dart/compiler/MockArtifactProvider.java
(right):

http://codereview.chromium.org/9464044/diff/1/compiler/javatests/com/google/d...
compiler/javatests/com/google/dart/compiler/MockArtifactProvider.java:79:
Artifact found = artifacts.remove(keyFor(name, part, ext));
unused assignment

http://codereview.chromium.org/9464044/diff/1/compiler/javatests/com/google/d...
File
compiler/javatests/com/google/dart/compiler/end2end/inc/IncrementalCompilationWithPrefixTest.java
(right):

http://codereview.chromium.org/9464044/diff/1/compiler/javatests/com/google/d...
compiler/javatests/com/google/dart/compiler/end2end/inc/IncrementalCompilationWithPrefixTest.java:122:
// didWrite("my.prefixed.app.dart", EXTENSION_APP_JS);
why not remove?