compare to null with == (#21944)

* compare to null with ==

* address review comments

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

@@ -83,11 +83,10 @@ class BoxDecoration extends Decoration {
     this.backgroundBlendMode,
     this.shape = BoxShape.rectangle,
   }) : assert(shape != null),
-       // TODO(mattcarroll): Use "backgroundBlendMode == null" when https://github.com/dart-lang/sdk/issues/34180 is in.
        assert(
-         identical(backgroundBlendMode, null) || color != null || gradient != null,
+         backgroundBlendMode == null || color != null || gradient != null,
          'backgroundBlendMode applies to BoxDecoration\'s background color or '
-         'gradient, but no color or gradient were provided.'
+         'gradient, but no color or gradient was provided.'
        );
 
   @override

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

@@ -246,9 +246,7 @@ class TextStyle extends Diagnosticable {
     String package,
   }) : fontFamily = package == null ? fontFamily : 'packages/$package/$fontFamily',
        assert(inherit != null),
-       // TODO(dnfield): once https://github.com/dart-lang/sdk/issues/33408 is finished, this can be replaced with
-       // assert(color == null || foreground == null, _kColorForegroundWarning);
-       assert(identical(color, null) || identical(foreground, null), _kColorForegroundWarning);
+       assert(color == null || foreground == null, _kColorForegroundWarning);
 
 
   /// Whether null values are replaced with their value in an ancestor text