dirty check should be one fixed-point per rAF

[DartBot]

<img src="https://avatars.githubusercontent.com/u/1081711?v=3" align="left" width="96" height="96"hspace="10"> Issue by jmesserly
Originally opened as dart-lang/sdk#19136

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Observable.dirtyCheck is triggering after every async operation. This can get very expensive. Better would be to batch them so we limit to 1 per rAF

discussion here:
https://github.com/Polymer/platform-dev/issues/7

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="48" height="48"hspace="10"> Comment by sigmundch

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Added this to the 1.6 milestone.
Removed Priority-Unassigned label.
Added Priority-Medium label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="48" height="48"hspace="10"> Comment by sigmundch

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Removed this from the 1.6 milestone.
Added Polymer-P-1 label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="48" height="48"hspace="10"> Comment by sigmundch

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Removed Polymer-P-1 label.
Added Polymer-Milestone-Next label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="48" height="48"hspace="10"> Comment by sigmundch

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Added PolymerMilestone-Next label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="48" height="48"hspace="10"> Comment by sigmundch

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Removed Polymer-Milestone-Next label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/1081711?v=3" align="left" width="48" height="48"hspace="10"> Comment by jmesserly

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moved