Provide Point+Size, Point-Point, Point.toSize(), and Rect.toPoint().

Also some minor code cleanup in affected files and nearby files.

R=mpcomplete@chromium.org

Review URL: https://codereview.chromium.org/1160453006

engine/core/painting/Color.dart

@@ -16,6 +16,7 @@ class Color {
               ((b & 0xff) << 0));
 
   bool operator ==(other) => other is Color && _value == other._value;
+
   int get hashCode => _value.hashCode;
   String toString() => "Color(0x${_value.toRadixString(16).padLeft(8, '0')})";
 }

engine/core/painting/Point.dart

@@ -11,10 +11,13 @@ class Point {
 
   const Point(this.x, this.y);
 
-  bool operator ==(other) {
-    if (!(other is Point)) return false;
-    return x == other.x && y == other.y;
-  }
+  bool operator ==(other) => other is Point && x == other.x && y == other.y;
+  Size operator -(Point other) => new Size(x - other.x, y - other.y);
+  Point operator +(Size size) => new Point(x + size.width, y + size.height);
+
+  // does the equivalent of "return this - Point(0,0)"
+  Size toSize() => new Size(x, y);
+
   int get hashCode {
     int result = 373;
     result = 37 * result + x.hashCode;

engine/core/painting/Rect.dart

@@ -44,18 +44,22 @@ class Rect {
 
   // Rects are inclusive of the top and left edges but exclusive of the bottom
   // right edges.
-  bool contains(Point point) =>
-      point.x >= left && point.x < right && point.y >= top && point.y < bottom;
+  bool contains(Point point) {
+    return point.x >= left && point.x < right && point.y >= top && point.y < bottom;
+  }
 
   bool operator ==(other) {
-    if (!(other is Rect)) return false;
+    if (other is! Rect)
+      return false;
     for (var i = 0; i < 4; ++i) {
-      if (_value[i] != other._value[i]) return false;
+      if (_value[i] != other._value[i])
+        return false;
     }
     return true;
   }
+
   int get hashCode {
     return _value.fold(373, (value, item) => (37 * value + item.hashCode));
   }
-  String toString() => "Rect.LTRB($left, $top, $right, $bottom)";
+  String toString() => "Rect.fromLTRB($left, $top, $right, $bottom)";
 }

engine/core/painting/Size.dart

@@ -5,6 +5,7 @@
 part of dart.sky;
 
 /// Holds a 2D floating-point size.
+/// Think of this as a vector from Point(0,0) to Point(size.width, size.height)
 class Size {
   final double width;
   final double height;
@@ -16,10 +17,11 @@ class Size {
   const Size.fromWidth(this.width) : height = double.INFINITY;
   const Size.fromHeight(this.height) : width = double.INFINITY;
 
-  bool operator ==(other) {
-    if (!(other is Size)) return false;
-    return width == other.width && height == other.height;
-  }
+  bool operator ==(other) => other is Size && width == other.width && height == other.height;
+
+  // does the equivalent of "return new Point(0,0) + this"
+  Point toPoint() => new Point(this.width, this.height);
+
   int get hashCode {
     int result = 373;
     result = 37 * result + width.hashCode;