Add some missing docs (#4772)

These docs complete the dartdocs for their respective libraries.

packages/flutter/lib/src/cassowary/solver.dart

@@ -145,6 +145,10 @@ class _Row {
   }
 }
 
+/// Solves cassowary constraints.
+///
+/// Typically clients will create a solver, [addConstraints], and then call
+/// [flushUpdates] to actually solve the constraints.
 class Solver {
   final Map<Constraint, _Tag> _constraints = new Map<Constraint, _Tag>();
   final Map<_Symbol, _Row> _rows = new Map<_Symbol, _Row>();
@@ -153,6 +157,9 @@ class Solver {
   final List<_Symbol> _infeasibleRows = new List<_Symbol>();
   final _Row _objective = new _Row(0.0);
   _Row _artificial = new _Row(0.0);
+
+  /// A monotonically increasing value that indicates how many times the solver
+  /// has iterated.
   int tick = 1;
 
   /// Attempts to add the constraints in the list to the solver. If it cannot

packages/flutter_sprites/lib/src/sprite_box.dart

@@ -85,6 +85,7 @@ class SpriteBox extends RenderBox {
   Duration _lastTimeStamp;
   double _frameRate = 0.0;
 
+  /// An instantaneous estimate of the number of frames per second this sprite box is producing.
   double get frameRate => _frameRate;
 
   // Transformation mode

packages/flutter_sprites/lib/src/textured_line.dart

@@ -25,6 +25,7 @@ class TexturedLine extends Node {
 
 /// Draws a polyline to a [Canvas] from a list of points using the provided [Texture].
 class TexturedLinePainter {
+  /// Creates a painter that draws a polyline with a texture.
   TexturedLinePainter(this._points, this.colors, this.widths, [Texture texture, this.textureStops]) {
     this.texture = texture;
   }

packages/flutter_test/lib/src/controller.dart

@@ -17,8 +17,10 @@ import 'test_pointer.dart';
 ///
 /// For a variant of this class suited specifically for unit tests, see [WidgetTester].
 class WidgetController {
+  /// Creates a widget controller that uses the given binding.
   WidgetController(this.binding);
 
+  /// A reference to the current instance of the binding.
   final WidgetsBinding binding;
 
   // FINDER API