Handle unnecessary_non_null_assertion and invalid_null_aware_operator (#2554)

It looks like the environment field is correctly analyzed as never
possibly null with a newer version of Dart. This fixes Dart CI.

Author: srujzs

dwds/test/build/min_sdk_test.dart

@@ -19,7 +19,7 @@ void main() {
     sdkVersion = Version(sdkVersion.major, sdkVersion.minor, 0);
 
     final sdkConstraint = VersionConstraint.compatibleWith(sdkVersion);
-    final pubspecSdkConstraint = pubspec.environment?['sdk'];
+    final pubspecSdkConstraint = pubspec.environment['sdk'];
     expect(pubspecSdkConstraint, isNotNull);
     expect(
       sdkConstraint.allowsAll(pubspecSdkConstraint!),

webdev/test/build/min_sdk_test.dart

@@ -19,7 +19,7 @@ void main() {
     sdkVersion = Version(sdkVersion.major, sdkVersion.minor, 0);
 
     final sdkConstraint = VersionConstraint.compatibleWith(sdkVersion);
-    final pubspecSdkConstraint = pubspec.environment!['sdk']!;
+    final pubspecSdkConstraint = pubspec.environment['sdk']!;
     expect(sdkConstraint.allowsAll(pubspecSdkConstraint), true,
         reason:
             'Min sdk constraint is outdated. Please update SDK constraint in '