Explicitly specify linear curves (#3973)

We now required the curve parameter for CurvedAnimation.

packages/flutter/lib/src/material/popup_menu.dart

@@ -384,6 +384,7 @@ class _PopupMenuRoute<T> extends PopupRoute<T> {
   Animation<double> createAnimation() {
     return new CurvedAnimation(
       parent: super.createAnimation(),
+      curve: Curves.linear,
       reverseCurve: new Interval(0.0, _kMenuCloseIntervalEnd)
     );
   }

packages/flutter/lib/src/material/toggleable.dart

@@ -46,7 +46,8 @@ abstract class RenderToggleable extends RenderConstrainedBox implements Semantic
       value: _value ? 1.0 : 0.0
     );
     _position = new CurvedAnimation(
-      parent: _positionController
+      parent: _positionController,
+      curve: Curves.linear
     )..addListener(markNeedsPaint)
      ..addStatusListener(_handlePositionStateChanged);