"pub serve" should lazily compile dart2js

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="96" height="96"hspace="10"> Issue by nex3
Originally opened as dart-lang/sdk#16076

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Currently, "pub serve" will compile dart2js for each Dart entrypoint it knows about. Issue #805 means that this will soon expand to include all tests and examples in an app. Since dart2js is currently quite slow, compiling all of these entrypoints in sequence will mean that the entrypoint the user is interested in will likely languish for a long time -- potentially tens of minutes for packages with many tests.

Until dart2js becomes much faster, the best way to deal with this is to only start compiling Dart entrypoints once they're requested. After that, they'll continue being recompiled as soon as the user edits an applicable source file.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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Marked this as blocking dart-lang/sdk#15830.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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Marked this as being blocked by dart-archive/barback#26.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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Added Started label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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Fixed by dart-lang/sdk@5fcc295.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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Added Fixed label.