make span non-nullable

pkgs/yaml/lib/src/yaml_node.dart

diff --git a/pkgs/yaml/lib/src/yaml_node.dart b/pkgs/yaml/lib/src/yaml_node.dart
index c8f294b..a02ea0f 100644
--- a/pkgs/yaml/lib/src/yaml_node.dart
+++ b/pkgs/yaml/lib/src/yaml_node.dart
@@ -26,9 +26,9 @@
   ///
   /// [SourceSpan.message] can be used to produce a human-friendly message about
   /// this node.
-  SourceSpan? get span => _span;
+  SourceSpan get span => _span;
 
-  SourceSpan? _span;
+  late SourceSpan _span;
 
   /// The inner value of this node.
   ///

pkgs/yaml/test/yaml_test.dart

diff --git a/pkgs/yaml/test/yaml_test.dart b/pkgs/yaml/test/yaml_test.dart
index 3633a5e..a0fb8f1 100644
--- a/pkgs/yaml/test/yaml_test.dart
+++ b/pkgs/yaml/test/yaml_test.dart
@@ -68,16 +68,16 @@
 - 123
 ''') as YamlList;
 
-    expect(yaml.span!.start.line, equals(0));
-    expect(yaml.span!.start.column, equals(0));
-    expect(yaml.span!.end.line, equals(3));
-    expect(yaml.span!.end.column, equals(0));
+    expect(yaml.span.start.line, equals(0));
+    expect(yaml.span.start.column, equals(0));
+    expect(yaml.span.end.line, equals(3));
+    expect(yaml.span.end.column, equals(0));
 
     var map = yaml.nodes.first as YamlMap;
-    expect(map.span!.start.line, equals(0));
-    expect(map.span!.start.column, equals(2));
-    expect(map.span!.end.line, equals(2));
-    expect(map.span!.end.column, equals(0));
+    expect(map.span.start.line, equals(0));
+    expect(map.span.start.column, equals(2));
+    expect(map.span.end.line, equals(2));
+    expect(map.span.end.column, equals(0));
 
     var key = map.nodes.keys.first;
     expect(key.span.start.line, equals(0));
@@ -86,16 +86,16 @@
     expect(key.span.end.column, equals(5));
 
     var value = map.nodes.values.first;
-    expect(value.span!.start.line, equals(1));
-    expect(value.span!.start.column, equals(4));
-    expect(value.span!.end.line, equals(1));
-    expect(value.span!.end.column, equals(7));
+    expect(value.span.start.line, equals(1));
+    expect(value.span.start.column, equals(4));
+    expect(value.span.end.line, equals(1));
+    expect(value.span.end.column, equals(7));
 
     var scalar = yaml.nodes.last;
-    expect(scalar.span!.start.line, equals(2));
-    expect(scalar.span!.start.column, equals(2));
-    expect(scalar.span!.end.line, equals(2));
-    expect(scalar.span!.end.column, equals(5));
+    expect(scalar.span.start.line, equals(2));
+    expect(scalar.span.start.column, equals(2));
+    expect(scalar.span.end.line, equals(2));
+    expect(scalar.span.end.column, equals(5));
   });
 
   // The following tests are all taken directly from the YAML spec