Fix off-by-one fromRGBO alpha value calculation (flutter#13777)

Constructing colors using `fromRGBO` should return the same values as the CSS
`rgba()` notation.  rgba(0, 0, 255, 0.5) is the same as `#0000ff80`

However `fromRGBO` sometimes creates a color with an off-by-one alpha value:

    expect(Color.fromRGBO(0, 0, 255, 0.5), Color(0x800000ff));

Expected: Color:<Color(0x800000ff)>
  Actual: Color:<Color(0x7f0000ff)>

If we use `withOpacity` to create the same color, it returns the correct color:

    expect(Color.fromRGBO(0, 0, 255, 1).withOpacity(0.5), Color(0x800000ff));

This should also be changed in lib/web_ui/lib/src/ui/painting.dart in a
followup change.

lib/ui/painting.dart

@@ -129,17 +129,17 @@ class Color {
   /// * `r` is [red], from 0 to 255.
   /// * `g` is [green], from 0 to 255.
   /// * `b` is [blue], from 0 to 255.
-  /// * `opacity` is alpha channel of this color as a double, with 0.0 being
+  /// * `opacity` is the alpha channel of this color as a double, with 0.0 being
   ///   transparent and 1.0 being fully opaque.
   ///
   /// Out of range values are brought into range using modulo 255.
   ///
   /// See also [fromARGB], which takes the opacity as an integer value.
   const Color.fromRGBO(int r, int g, int b, double opacity) :
-    value = ((((opacity * 0xff ~/ 1) & 0xff) << 24) |
-              ((r                    & 0xff) << 16) |
-              ((g                    & 0xff) << 8)  |
-              ((b                    & 0xff) << 0)) & 0xFFFFFFFF;
+    value = (((((opacity * 0xff + 0.5) ~/ 1) & 0xff) << 24) | // Since colors are canonicalized we need to round the alpha value manually
+              ((r                            & 0xff) << 16) |
+              ((g                            & 0xff) << 8)  |
+              ((b                            & 0xff) << 0)) & 0xFFFFFFFF;
 
   /// A 32 bit value representing this color.
   ///

testing/dart/color_test.dart

@@ -55,6 +55,12 @@ void main() {
     expect(const NotAColor(123), equals(const NotAColor(123)));
   });
 
+  test('Color.fromRGBO', () {
+    expect(const Color.fromRGBO(0, 0, 0, 1.0), const Color(0xFF000000));
+    expect(const Color.fromRGBO(0, 0, 0, 0.5), const Color(0x80000000));
+    expect(const Color.fromRGBO(0, 0, 0, 0.0), const Color(0x00000000));
+  });
+
   test('Color.lerp', () {
     expect(
       Color.lerp(const Color(0x00000000), const Color(0xFFFFFFFF), 0.0),