Issue 1:
To reproduce:

Launch the "Drop Down Button" from the material_gallery example

Press the menu button

android: I/flutter : Exception caught while building _DropDownMenu
android: I/flutter : 'packages/flutter/src/widgets/basic.dart': Failed assertion: line 749: '() {
android: I/flutter : "Positioned must placed inside a Stack";
android: I/flutter : return ancestor is Stack;
android: I/flutter : }' is not true.
android: I/flutter : Stack trace:
android: I/flutter : #0 _AssertionError._throwNew (dart:core-patch/errors_patch.dart:27)
android: I/flutter : #1 Positioned.debugValidateAncestor (packages/flutter/src/widgets/basic.dart:749)
android: I/flutter : #2 ParentDataElement.mount.<anonymous closure> (packages/flutter/src/widgets/framework.dart:1226)
android: I/flutter : #3 ParentDataElement.mount (packages/flutter/src/widgets/framework.dart:1216)
android: I/flutter : #4 Element._inflateWidget (packages/flutter/src/widgets/framework.dart:770)
android: I/flutter : #5 Element.updateChild (packages/flutter/src/widgets/framework.dart:649)
android: I/flutter : #6 ComponentElement.performRebuild (packages/flutter/src/widgets/framework.dart:1082)
android: I/flutter : #7 BuildableElement.rebuild (packages/flutter/src/widgets/framework.dart:1011)
android: I/flutter : #8 ComponentElement._firstBuild (packages/flutter/src/widgets/framework.dart:1057)
android: I/flutter : #9 StatefulComponentElement._firstBuild (packages/flutter/src/widgets/framework.dart:1151)
android: I/flutter : #10 ComponentElement.mount (packages/flutter/src/widgets/framework.dart:1052)
android: I/flutter : #11 Element._inflateWidget (packages/flutter/src/widgets/framework.dart:770)
android: I/flutter : #12 Element.updateChild (packages/flutter/src/widgets/framework.dart:649)
android: I/flutter : #13 ComponentElement.performRebuild (packages/flutter/src/widgets/framework.dart:1082)
android: I/flutter : #14 BuildableElement.rebuild (packages/flutter/src/widgets/framework.dart:1011)
android: I/flutter : #15 ComponentElement._firstBuild (packages/flutter/src/widgets/framework.dart:1057)
android: I/flutter : #16 ComponentElement.mount (packages/flutter/src/widgets/framework.dart:1052)
android: I/flutter : #17 Element._inflateWidget (packages/flutter/src/widgets/framework.dart:770)
android: I/flutter : #18 Element.updateChild (packages/flutter/src/widgets/framework.dart:649)
android: I/flutter : #19 ComponentElement.performRebuild (packages/flutter/src/widgets/framework.dart:1082)
android: I/flutter : #20 BuildableElement.rebuild (packages/flutter/src/widgets/framework.dart:1011)
android: I/flutter : #21 ComponentElement._firstBuild (packages/flutter/src/widgets/framework.dart:1057)
android: I/flutter : #22 ComponentElement.mount (packages/flutter/src/widgets/framework.dart:1052)
android: I/flutter : #23 Element._inflateWidget (packages/flutter/src/widgets/framework.dart:770)
android: I/flutter : #24 Element.updateChild (packages/flutter/src/widgets/framework.dart:649)
android: I/flutter : #25 OneChildRenderObjectElement.mount (packages/flutter/src/widgets/framework.dart:1572)
android: I/flutter : #26 Element._inflateWidget (packages/flutter/src/widgets/framework.dart:770)
android: I/flutter : #27 Element.updateChild (packages/flutter/src/widgets/framework.dart:649)
android: I/flutter : #28 ComponentElement.performRebuild (packages/flutter/src/widgets/framework.dart:1082)
android: I/flutter : #29 BuildableElement.rebuild (packages/flutter/src/widgets/framework.dart:1011)
android: I/flutter : #30 ComponentElement._firstBuild (packages/flutter/src/widgets/framework.dart:1057)
android: I/flutter : #31 ComponentElement.mount (packages/flutter/src/widgets/framework.dart:1052)
android: I/flutter : #32 Element._inflateWidget (packages/flutter/src/widgets/framework.dart:770)
android: I/flutter : #33 Element.updateChild (packages/flutter/src/widgets/framework.dart:649)
android: I/flutter : #34 ComponentElement.performRebuild (packages/flutter/src/widgets/framework.dart:1082)
android: I/flutter : #35 BuildableElement.rebuild (packages/flutter/src/widgets/framework.dart:1011)
android: I/flutter : #36 ComponentElement._firstBuild (packages/flutter/src/widgets/framework.dart:1057)
android: I/flutter : #37 StatefulComponentElement._firstBuild (packages/flutter/src/widgets/framework.dart:1151)
android: I/flutter : #38 ComponentElement.mount (packages/flutter/src/widgets/framework.dart:1052)
android: I/flutter : #39 Element._inflateWidget (packages/flutter/src/widgets/framework.dart:770)
android: I/flutter : #40 Element.updateChild (packages/flutter/src/widgets/framework.dart:649)
android: I/flutter : #41 OneChildRenderObjectElement.mount (packages/flutter/src/widgets/framework.dart:1572)
android: I/flutter : #42 Element._inflateWidget (packages/flutter/src/widgets/framework.dart:770)
android: I/flutter : #43 Element.updateChild (packages/flutter/src/widgets/framework.dart:649)

Issue 2: