migrate physics to nullsafety (#61941)

packages/flutter/lib/physics.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 /// Simple one-dimensional physics simulations, such as springs, friction, and
 /// gravity, for use in user interface animations.
 ///

packages/flutter/lib/src/physics/clamped_simulation.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'simulation.dart';
 
 /// A simulation that applies limits to another simulation.

packages/flutter/lib/src/physics/friction_simulation.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'dart:math' as math;
 
 import 'simulation.dart';

packages/flutter/lib/src/physics/gravity_simulation.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'simulation.dart';
 
 // Examples can assume:

packages/flutter/lib/src/physics/simulation.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:flutter/foundation.dart';
 
 import 'tolerance.dart';

packages/flutter/lib/src/physics/spring_simulation.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'dart:math' as math;
 
 import 'package:flutter/foundation.dart';
@@ -20,9 +18,9 @@ class SpringDescription {
   ///
   /// See [mass], [stiffness], and [damping] for the units of the arguments.
   const SpringDescription({
-    this.mass,
-    this.stiffness,
-    this.damping,
+    required this.mass,
+    required this.stiffness,
+    required this.damping,
   });
 
   /// Creates a spring given the mass (m), stiffness (k), and damping ratio (ζ).
@@ -33,8 +31,8 @@ class SpringDescription {
   /// See [mass] and [stiffness] for the units for those arguments. The damping
   /// ratio is unitless.
   SpringDescription.withDampingRatio({
-    this.mass,
-    this.stiffness,
+    required this.mass,
+    required this.stiffness,
     double ratio = 1.0,
   }) : damping = ratio * 2.0 * math.sqrt(mass * stiffness);
 

packages/flutter/lib/src/physics/tolerance.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 /// Structure that specifies maximum allowable magnitudes for distances,
 /// durations, and velocity differences to be considered equal.
 class Tolerance {

packages/flutter/lib/src/physics/utils.dart

@@ -2,14 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 /// Whether two doubles are within a given distance of each other.
 ///
 /// The `epsilon` argument must be positive and not null.
 /// The `a` and `b` arguments may be null. A null value is only considered
 /// near-equal to another null value.
-bool nearEqual(double a, double b, double epsilon) {
+bool nearEqual(double? a, double? b, double epsilon) {
   assert(epsilon != null);
   assert(epsilon >= 0.0);
   if (a == null || b == null)