Renamed Step curve to Threshold. (#5755)

In the interest of using the name in the case of Material steppers,
this commit renames Step curve to Threshold.

packages/flutter/lib/src/animation/curves.dart

@@ -112,11 +112,11 @@ class Interval extends Curve {
 }
 
 /// A curve that is 0.0 until it hits the threshold, then it jumps to 1.0.
-class Step extends Curve {
-  /// Creates a step cruve.
+class Threshold extends Curve {
+  /// Creates a threshold curve.
   ///
   /// The [threshold] argument must not be null.
-  const Step(this.threshold);
+  const Threshold(this.threshold);
 
   /// The value before which the curve is 0.0 and after which the curve is 1.0.
   ///

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

@@ -125,7 +125,7 @@ class _DropDownMenuState<T> extends State<_DropDownMenu<T>> {
     _resize = new CurvedAnimation(
       parent: config.route.animation,
       curve: const Interval(0.25, 0.5),
-      reverseCurve: const Step(0.0)
+      reverseCurve: const Threshold(0.0)
     );
   }
 
@@ -145,7 +145,7 @@ class _DropDownMenuState<T> extends State<_DropDownMenu<T>> {
     for (int itemIndex = 0; itemIndex < route.items.length; ++itemIndex) {
       CurvedAnimation opacity;
       if (itemIndex == route.selectedIndex) {
-        opacity = new CurvedAnimation(parent: route.animation, curve: const Step(0.0));
+        opacity = new CurvedAnimation(parent: route.animation, curve: const Threshold(0.0));
       } else {
         final double start = (0.5 + (itemIndex + 1) * unit).clamp(0.0, 1.0);
         final double end = (start + 1.5 * unit).clamp(0.0, 1.0);

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

@@ -152,7 +152,7 @@ class SnackBar extends StatelessWidget {
     if (action != null)
       children.add(action);
     CurvedAnimation heightAnimation = new CurvedAnimation(parent: animation, curve: _snackBarHeightCurve);
-    CurvedAnimation fadeAnimation = new CurvedAnimation(parent: animation, curve: _snackBarFadeCurve, reverseCurve: const Step(0.0));
+    CurvedAnimation fadeAnimation = new CurvedAnimation(parent: animation, curve: _snackBarFadeCurve, reverseCurve: const Threshold(0.0));
     ThemeData theme = Theme.of(context);
     return new ClipRect(
       child: new AnimatedBuilder(

packages/flutter/test/animation/curves_test.dart

@@ -25,8 +25,8 @@ void main() {
     expect(flippedEase, hasOneLineDescription);
   });
 
-  test('Step has a step', () {
-    Curve step = new Step(0.25);
+  test('Threshold has a threshold', () {
+    Curve step = new Threshold(0.25);
     expect(step.transform(0.0), 0.0);
     expect(step.transform(0.24), 0.0);
     expect(step.transform(0.25), 1.0);