Fix the exception caused by <spark-button> in #editor-dialog in Dartium 33

From bug #1078:

Dartium 33 throws the following exception on launch and on every click on a file whose format is not supported by Spark by default:

type 'HtmlElement' is not a subtype of type 'SparkButton' of 'button'.
  undefined (undefined:0:0)
#0      AceManager.createDialog (chrome-extension://kcjgcakhgelcejampmijgkjkadfcncjl/lib/ace.dart:335:46)
#1      EditorManager.activate (chrome-extension://kcjgcakhgelcejampmijgkjkadfcncjl/lib/editors.dart:365:31)
#2      AceEditorTab.activate (chrome-extension://kcjgcakhgelcejampmijgkjkadfcncjl/lib/editor_area.dart:48:22)
#3      TabView.selectedTab= (chrome-extension://kcjgcakhgelcejampmijgkjkadfcncjl/lib/ui/widgets/tabview.dart:180:34)
#4      TabView.replace (chrome-extension://kcjgcakhgelcejampmijgkjkadfcncjl/lib/ui/widgets/tabview.dart:213:9)
#5      EditorArea.replace (chrome-extension://kcjgcakhgelcejampmijgkjkadfcncjl/lib/editor_area.dart:136:25)
#6      EditorArea.selectFile (chrome-extension://kcjgcakhgelcejampmijgkjkadfcncjl/lib/editor_area.dart:183:16)
#7      Spark.selectInEditor (chrome-extension://kcjgcakhgelcejampmijgkjkadfcncjl/spark.dart:538:28)
#8      FilesController.treeViewSelectedChanged (chrome-extension://kcjgcakhgelcejampmijgkjkadfcncjl/lib/ui/files_controller.dart:181:33)
#9      TreeView.listViewSelectedChanged (chrome-extension://kcjgcakhgelcejampmijgkjkadfcncjl/lib/ui/widgets/treeview.dart:291:38)
#10     ListView._setSelection (chrome-extension://kcjgcakhgelcejampmijgkjkadfcncjl/lib/ui/widgets/listview.dart:276:38)
#11     ListView._onClicked (chrome-extension://kcjgcakhgelcejampmijgkjkadfcncjl/lib/ui/widgets/listview.dart:217:20)
#12     ListView.reloadData. (chrome-extension://kcjgcakhgelcejampmijgkjkadfcncjl/lib/ui/widgets/listview.dart:161:19)
#13     dirtyCheckZoneSpec.wrapUnaryCallback. (package:observe/src/dirty_check.dart:127:15)
#14     _rootRunUnary (dart:async/zone.dart:721)
#15     _rootRunUnary (dart:async/zone.dart:722)
#16     _rootRunUnary (dart:async/zone.dart:722)
#17     _ZoneDelegate.runUnary (dart:async/zone.dart:449)
#18     _CustomizedZone.runUnary (dart:async/zone.dart:654)
#19     _BaseZone.runUnaryGuarded (dart:async/zone.dart:569)
#20     _BaseZone.bindUnaryCallback. (dart:async/zone.dart:595)

  undefined (undefined:0:0)
This is caused by <spark-button> used in the #editor-dialog template in spark_polymer.html. It used to work, although intermittently (see #976 ). Now it's completely broken. It looks like <spark-button> doesn't get a chance to get upgraded in time or something like that, so it's returned as HtmlElement. I think this mixing of manual <template> handling and using a Polymer element in the template might be the culprit of this and #976 . Switching to <button> works, but only with html.HtmlElement at ace.dart:335; html.ButtonElement there prints the same stack trace with the same message about SparkButton, which is completely bizarre. I didn't investigate any further.
Merge pull request #1079 from ussuri/fix-editor-dialog-exception-in-dartium33

Fix the exception caused by <spark-button> in #editor-dialog in Dartium 33