clean up ChangeRecord API

[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#14157

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* Observable.changes should only be property changes.

ObservableList will have "listChanges" property which includes the ListChangeRecords.

Splitting this allows the consumer to know what kind of records it is getting, and makes the API constraints more clear.

* remove ChangeRecord base class

* add "target" to List/Map change record

* add old/new values to ListChangeRecord? See http://wiki.ecmascript.org/doku.php?id=harmony:observe#array.observe

[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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Removed this from the M8 milestone.
Added this to the Later milestone.

[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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first step here:
https://codereview.chromium.org/53743002/

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Set owner to @jmesserly.
Added Started 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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Update: all issues are fixed except splitting out MapChangeRecord and adding "object" to it.

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Removed the owner.
Added Accepted 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 Later milestone.
Added this to the M9 milestone.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/8616782?v=3" align="left" width="48" height="48"hspace="10"> Comment by clayberg

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Removed this from the M9 milestone.
Added this to the 1.1 milestone.

[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 1.1 milestone.
Added this to the 1.2 milestone.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/3276024?v=3" align="left" width="48" height="48"hspace="10"> Comment by anders-sandholm

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Removed Library-Observe label.
Added Pkg-Observe 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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Removed Area-Polymer label.
Added Area-Pkg label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2156198?v=3" align="left" width="48" height="48"hspace="10"> Comment by kasperl

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Removed this from the 1.2 milestone.
Added this to the 1.6 milestone.

[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 Priority-Unassigned label.
Added Priority-Low 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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Removed Polymer-Milestone-Next label.
Added PolymerMilestone-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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#3

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Added MovedToGithub label.