Make build incremental

Make build incremental

- Make sure observatory's index.html is touched.
- Switch generate_snapshot_bin to have only one output (a timestamp file)

Fixes #24220

R=iposva@google.com, ricow@google.com

Committed: https://github.com/dart-lang/sdk/commit/021d7c0aa539c47d868749f6cf1a8aa538f8b19f

[Cutch, 2015-08-27 14:49:10]

johnmccutchan@google.com changed reviewers:
+ asiva@google.com, iposva@google.com, ricow@google.com

[ricow1, 2015-08-27 15:00:26]

lgtm

https://codereview.chromium.org/1304953004/diff/1/runtime/tools/create_snapsh...
File runtime/tools/create_snapshot_bin.py (right):

https://codereview.chromium.org/1304953004/diff/1/runtime/tools/create_snapsh...
runtime/tools/create_snapshot_bin.py:83: open(options.timestamp_file,
'w').close()
does this actually change the timestamp if the file exists?

if the file is already there you can do os.utime(options.timestamp_file, None)

https://codereview.chromium.org/1304953004/diff/1/tools/observatory_tool.py
File tools/observatory_tool.py (right):

https://codereview.chromium.org/1304953004/diff/1/tools/observatory_tool.py#n...
tools/observatory_tool.py:99: index_file = os.path.join(output_dir, 'web',
'index.html')
you could pass this in, but I by no means have strong feelings about it

[Ivan Posva, 2015-08-27 15:24:58]

LGTM -ip

[Cutch, 2015-08-27 18:14:18]

Committed patchset #2 (id:20001) manually as
021d7c0aa539c47d868749f6cf1a8aa538f8b19f (presubmit successful).