Improve the save operation of DartEditor

[DartBot]

This issue was originally filed by @tomyeh

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As the application getting bigger, the save operation of DartEditor is coming unacceptable - from tens seconds to a couple of minutes! To be productive, I was forced to use other text editor and invoke frogc manually.

I know it is tough and the Dart team must work on JDT-like incremental compilation. However, before the performance issue is really solved, I suggest to introduce an option that allows the user to turn off the compilation for each save. Then, the user can invoke the compilation at any proper moment he prefers.

[anders-sandholm]

AFAIK, we're looking at both speeding up things as well as making the operation asynchronous.

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Added Area-Editor, Triaged labels.

[danrubel]

We are in the process of making analysis more performant.
You can try it out by following the instructions here...
http://code.google.com/p/dart/issues/detail?id=2297

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Added Duplicate label.
Marked as being merged into #2297.