Add and document and explicit toARGB32. (flutter/engine#56329)

Closes https://github.com/flutter/flutter/issues/157128.

engine/src/flutter/lib/ui/painting.dart

@@ -197,8 +197,26 @@ class Color {
   /// * Bits 16-23 are the red value.
   /// * Bits 8-15 are the green value.
   /// * Bits 0-7 are the blue value.
-  @Deprecated('Use component accessors like .r or .g.')
-  int get value {
+  @Deprecated('Use component accessors like .r or .g, or toARGB32 for an explicit conversion')
+  int get value => toARGB32();
+
+  /// Returns a 32-bit value representing this color.
+  ///
+  /// Unlike accessing the floating point equivalent channels individually
+  /// ([a], [r], [g], [b]), this method is intentionally _lossy_, and scales
+  /// each channel using `(channel * 255.0).round() & 0xff`.
+  ///
+  /// While useful for storing a 32-bit integer value, prefer accessing the
+  /// individual channels (and storing the double equivalent) where higher
+  /// precision is required.
+  ///
+  /// The bits are assigned as follows:
+  ///
+  /// * Bits 24-31 represents the [a] channel as an 8-bit unsigned integer.
+  /// * Bits 16-23 represents the [r] channel as an 8-bit unsigned integer.
+  /// * Bits 8-15 represents the [g] channel as an 8-bit unsigned integer.
+  /// * Bits 0-7 represents the [b] channel as an 8-bit unsigned integer.
+  int toARGB32() {
     return _floatToInt8(a) << 24 |
         _floatToInt8(r) << 16 |
         _floatToInt8(g) << 8 |

engine/src/flutter/lib/web_ui/lib/painting.dart

@@ -68,14 +68,15 @@ class Color {
     return (x * 255.0).round() & 0xff;
   }
 
-  int get value {
+  int get value => toARGB32();
+
+  int toARGB32() {
     return _floatToInt8(a) << 24 |
         _floatToInt8(r) << 16 |
         _floatToInt8(g) << 8 |
         _floatToInt8(b) << 0;
   }
 
-
   int get alpha => (0xff000000 & value) >> 24;
 
   double get opacity => alpha / 0xFF;

engine/src/flutter/testing/dart/color_test.dart

@@ -339,6 +339,11 @@ void main() {
     // Call base class member, make sure it uses overridden value.
     expect(color.red, 0xE0);
   });
+
+  test('toARGB32 converts to a 32-bit integer', () {
+    const Color color = Color.from(alpha: 0.1, red: 0.2, green: 0.3, blue: 0.4);
+    expect(color.toARGB32(), equals(0x1a334d66));
+  });
 }
 
 class DynamicColorClass extends Color {