Code cleanup for physics (#9193)

* Add tolerance argument to FrictionSimulation.
* Change FrictionSimulation.through to setting tolerance
  via the constructor rather than afterwards.
* Allow SimulationGroup constructor to take tolerance argument.
* Add a toString for SpringDescription.
* Add a toString for SpringSimulation.
* Push this change to BouncingScrollSimulation.

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

@@ -18,11 +18,13 @@ class FrictionSimulation extends Simulation {
   /// drag coefficient, a unitless value; the initial position, in the same
   /// length units as used for [x]; and the initial velocity, in the same
   /// velocity units as used for [dx].
-  FrictionSimulation(double drag, double position, double velocity)
-    : _drag = drag,
+  FrictionSimulation(double drag, double position, double velocity, {
+    Tolerance tolerance: Tolerance.defaultTolerance,
+  }) : _drag = drag,
        _dragLog = math.log(drag),
        _x = position,
-      _v = velocity;
+       _v = velocity,
+       super(tolerance: tolerance);
 
   /// Creates a new friction simulation with its fluid drag coefficient set so
   /// as to ensure that the simulation starts and ends at the specified
@@ -42,8 +44,9 @@ class FrictionSimulation extends Simulation {
     return new FrictionSimulation(
       _dragFor(startPosition, endPosition, startVelocity, endVelocity),
       startPosition,
-      startVelocity
-    )..tolerance = new Tolerance(velocity: endVelocity.abs());
+      startVelocity,
+      tolerance: new Tolerance(velocity: endVelocity.abs()),
+    );
   }
 
   final double _drag;

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

@@ -19,6 +19,8 @@ import 'utils.dart';
 /// by this group as they become active. This mean simulations should not be
 /// shared among different groups that are active at the same time.
 abstract class SimulationGroup extends Simulation {
+  /// Initializes the [tolerance] field for subclasses.
+  SimulationGroup({ Tolerance tolerance: Tolerance.defaultTolerance }) : super(tolerance: tolerance);
 
   /// The currently active simulation.
   ///

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

@@ -53,6 +53,9 @@ class SpringDescription {
   /// used for the value of the [mass] property, and T is the time unit used for
   /// driving the [SpringSimulation].
   final double damping;
+
+  @override
+  String toString() => '$runtimeType(mass: ${mass.toStringAsFixed(1)}, springConstant: ${springConstant.toStringAsFixed(1)}, damping: ${damping.toStringAsFixed(1)})';
 }
 
 /// The kind of spring solution that the [SpringSimulation] is using to simulate the spring.
@@ -114,6 +117,9 @@ class SpringSimulation extends Simulation {
     return nearZero(_solution.x(time), tolerance.distance) &&
            nearZero(_solution.dx(time), tolerance.velocity);
   }
+
+  @override
+  String toString() => '$runtimeType(end: $_endPosition, $type)';
 }
 
 /// A SpringSimulation where the value of [x] is guaranteed to have exactly the

packages/flutter/lib/src/widgets/scroll_simulation.dart

@@ -34,9 +34,11 @@ class BouncingScrollSimulation extends SimulationGroup {
     @required double leadingExtent,
     @required double trailingExtent,
     @required SpringDescription spring,
+    Tolerance tolerance: Tolerance.defaultTolerance,
   }) : _leadingExtent = leadingExtent,
        _trailingExtent = trailingExtent,
-       _spring = spring {
+       _spring = spring,
+       super(tolerance: tolerance) {
     assert(position != null);
     assert(velocity != null);
     assert(_leadingExtent != null);
@@ -73,15 +75,15 @@ class BouncingScrollSimulation extends SimulationGroup {
       if (position > _trailingExtent) {
         _isSpringing = true;
         _offset = intervalOffset;
-        _currentSimulation = new ScrollSpringSimulation(_spring, position, _trailingExtent, velocity);
+        _currentSimulation = new ScrollSpringSimulation(_spring, position, _trailingExtent, velocity, tolerance: tolerance);
         return true;
       } else if (position < _leadingExtent) {
         _isSpringing = true;
         _offset = intervalOffset;
-        _currentSimulation = new ScrollSpringSimulation(_spring, position, _leadingExtent, velocity);
+        _currentSimulation = new ScrollSpringSimulation(_spring, position, _leadingExtent, velocity, tolerance: tolerance);
         return true;
       } else if (_currentSimulation == null) {
-        _currentSimulation = new FrictionSimulation(0.135, position, velocity);
+        _currentSimulation = new FrictionSimulation(0.135, position, velocity, tolerance: tolerance);
         return true;
       }
     }