Remove suppression of null-related warnings

Since dart-lang/sdk#41905 has closed, this CL removes the suppression of
unnecessary_non_null_assertion and unnecessary_null_comparison.

Related issues: dart-lang/sdk#41905 (resolved)

lib/web_ui/analysis_options.yaml

@@ -12,8 +12,6 @@ analyzer:
     missing_required_param: warning
     missing_return: warning
     native_function_body_in_non_sdk_code: ignore
-    unnecessary_non_null_assertion: ignore
-    unnecessary_null_comparison: ignore
     todo: ignore
 
 linter:

lib/web_ui/lib/src/engine/canvaskit/text.dart

@@ -31,7 +31,6 @@ class CkParagraphStyle implements ui.ParagraphStyle {
           fontStyle,
           ellipsis,
         ) {
-    assert(skParagraphStyle != null);
     _textDirection = textDirection ?? ui.TextDirection.ltr;
     _fontFamily = fontFamily;
   }

lib/web_ui/lib/src/ui/painting.dart

@@ -528,12 +528,9 @@ class Shadow {
     this.color = const Color(_kColorDefault),
     this.offset = Offset.zero,
     this.blurRadius = 0.0,
-  })  : assert(color != null,
-            'Text shadow color was null.'), // ignore: unnecessary_null_comparison
-        assert(offset != null,
-            'Text shadow offset was null.'), // ignore: unnecessary_null_comparison
-        assert(blurRadius >= 0.0,
-            'Text shadow blur radius should be non-negative.');
+  })  : assert(color != null, 'Text shadow color was null.'), // ignore: unnecessary_null_comparison
+        assert(offset != null, 'Text shadow offset was null.'), // ignore: unnecessary_null_comparison
+        assert(blurRadius >= 0.0, 'Text shadow blur radius should be non-negative.');
 
   static const int _kColorDefault = 0xFF000000;
   final Color color;