Add const non-null asserts where required (#9939)

Also fixes a small typo.

packages/flutter/lib/src/gestures/drag_details.dart

@@ -100,6 +100,7 @@ class DragUpdateDetails {
     this.primaryDelta,
     @required this.globalPosition
   }) {
+    assert(delta != null);
     assert(primaryDelta == null
         || (primaryDelta == delta.dx && delta.dy == 0.0)
         || (primaryDelta == delta.dy && delta.dx == 0.0));

packages/flutter/lib/src/gestures/velocity_tracker.dart

@@ -13,7 +13,7 @@ class Velocity {
   /// Creates a velocity.
   ///
   /// The [pixelsPerSecond] argument must not be null.
-  const Velocity({ this.pixelsPerSecond });
+  const Velocity({ this.pixelsPerSecond }) : assert(pixelsPerSecond != null);
 
   /// A velocity that isn't moving at all.
   static const Velocity zero = const Velocity(pixelsPerSecond: Offset.zero);
@@ -87,12 +87,17 @@ class Velocity {
 ///    useful velocity operations.
 class VelocityEstimate {
   /// Creates a dimensional velocity estimate.
+  ///
+  /// [pixelsPerSecond], [confidence], [duration], and [offset] must not be null.
   const VelocityEstimate({
     this.pixelsPerSecond,
     this.confidence,
     this.duration,
     this.offset,
-  });
+  }) : assert(pixelsPerSecond != null),
+       assert(confidence != null),
+       assert(duration != null),
+       assert(offset != null);
 
   /// The number of pixels per second of velocity in the x and y directions.
   final Offset pixelsPerSecond;
@@ -116,7 +121,9 @@ class VelocityEstimate {
 }
 
 class _PointAtTime {
-  const _PointAtTime(this.point, this.time);
+  const _PointAtTime(this.point, this.time)
+      : assert(point != null),
+        assert(time != null);
 
   final Duration time;
   final Offset point;

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

@@ -40,7 +40,7 @@ class DataColumn {
     this.tooltip,
     this.numeric: false,
     this.onSort
-  });
+  }) : assert(label != null);
 
   /// The column heading.
   ///
@@ -92,7 +92,7 @@ class DataRow {
     this.selected: false,
     this.onSelectChanged,
     this.cells
-  });
+  }) : assert(cells != null);
 
   /// Creates the configuration for a row of a [DataTable], deriving
   /// the key from a row index.
@@ -103,7 +103,8 @@ class DataRow {
     this.selected: false,
     this.onSelectChanged,
     this.cells
-  }) : key = new ValueKey<int>(index);
+  }) : assert(cells != null),
+       key = new ValueKey<int>(index);
 
   /// A [Key] that uniquely identifies this row. This is used to
   /// ensure that if a row is added or removed, any stateful widgets
@@ -167,7 +168,7 @@ class DataCell {
     this.placeholder: false,
     this.showEditIcon: false,
     this.onTap,
-  });
+  }) : assert(child != null);
 
   /// A cell that has no content and has zero width and height.
   static final DataCell empty = new DataCell(new Container(width: 0.0, height: 0.0));

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

@@ -69,7 +69,11 @@ class IconButton extends StatelessWidget {
     this.disabledColor,
     @required this.onPressed,
     this.tooltip
-  }) : super(key: key);
+  }) : assert(iconSize != null),
+       assert(padding != null),
+       assert(alignment != null),
+       assert(icon != null),
+       super(key: key);
 
   /// The size of the icon inside the button.
   ///

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

@@ -137,6 +137,7 @@ class ListTile extends StatelessWidget {
   }) : assert(isThreeLine != null),
        assert(enabled != null),
        assert(selected != null),
+       assert(!isThreeLine || subtitle != null),
        super(key: key);
 
   /// A widget to display before the title.

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

@@ -17,7 +17,7 @@ abstract class MergeableMaterialItem {
   /// const constructors so that they can be used in const expressions.
   ///
   /// The argument is the [key], which must not be null.
-  const MergeableMaterialItem(this.key);
+  const MergeableMaterialItem(this.key) : assert(key != null);
 
   /// The key for this item of the list.
   ///

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

@@ -232,7 +232,8 @@ class DefaultTabController extends StatefulWidget {
     @required this.length,
     this.initialIndex: 0,
     @required this.child,
-  }) : super(key: key);
+  }) : assert(initialIndex != null),
+       super(key: key);
 
   /// The total number of tabs. Must be greater than one.
   final int length;

packages/flutter/lib/src/painting/box_painter.dart

@@ -993,7 +993,7 @@ class LinearGradient extends Gradient {
 ///  * [CustomPainter], which shows how to use the above sample code in a custom
 ///    painter.
 class RadialGradient extends Gradient {
-  /// Creates a radial graident.
+  /// Creates a radial gradient.
   ///
   /// The [colors] argument must not be null. If [stops] is non-null, it must
   /// have the same length as [colors].
@@ -1003,7 +1003,10 @@ class RadialGradient extends Gradient {
     this.colors,
     this.stops,
     this.tileMode: TileMode.clamp,
-  });
+  }) : assert(center != null),
+       assert(radius != null),
+       assert(colors != null),
+       assert(tileMode != null);
 
   /// The center of the gradient, as an offset into the unit square
   /// describing the gradient which will be mapped onto the paint box.
@@ -1265,7 +1268,7 @@ class DecorationImage {
     this.centerSlice,
     this.colorFilter,
     this.alignment,
-  });
+  }) : assert(image != null);
 
   /// The image to be painted into the decoration.
   final ImageProvider image;

packages/flutter/lib/src/painting/colors.dart

@@ -17,7 +17,11 @@ class HSVColor {
   ///
   /// All the arguments must not be null and be in their respective ranges. See
   /// the fields for each parameter for a description of their ranges.
-  const HSVColor.fromAHSV(this.alpha, this.hue, this.saturation, this.value);
+  const HSVColor.fromAHSV(this.alpha, this.hue, this.saturation, this.value)
+      : assert(alpha != null),
+        assert(hue != null),
+        assert(saturation != null),
+        assert(value != null);
 
   /// Alpha, from 0.0 to 1.0.
   final double alpha;

packages/flutter/lib/src/painting/text_style.dart

@@ -26,7 +26,7 @@ class TextStyle {
     this.decoration,
     this.decorationColor,
     this.decorationStyle
-  });
+  }) : assert(inherit != null);
 
   /// Whether null values are replaced with their value in an ancestor text style (e.g., in a [TextSpan] tree).
   final bool inherit;