Disable checking of contracts. (#183)

We've historically accomplished this with `-AsuppressWarnings`, but this
CL's solution may be more general. Also, it avoids running some code
entirely, which saves us from having to keep that code updated for the
very new JDKs that we run with. (It might also help performance if we're
lucky, and maybe it moves us a step closer to not needing JavaParser
(though that may be on the way out, anyway).)

src/main/java/com/google/jspecify/nullness/NullSpecAnnotatedTypeFactory.java

@@ -110,6 +110,10 @@
 import org.checkerframework.framework.type.typeannotator.TypeAnnotator;
 import org.checkerframework.framework.type.visitor.AnnotatedTypeVisitor;
 import org.checkerframework.framework.util.AnnotationFormatter;
+import org.checkerframework.framework.util.Contract.ConditionalPostcondition;
+import org.checkerframework.framework.util.Contract.Postcondition;
+import org.checkerframework.framework.util.Contract.Precondition;
+import org.checkerframework.framework.util.ContractsFromMethod;
 import org.checkerframework.framework.util.DefaultAnnotationFormatter;
 import org.checkerframework.framework.util.DefaultQualifierKindHierarchy;
 import org.checkerframework.framework.util.QualifierKindHierarchy;
@@ -919,6 +923,28 @@ public boolean applyConservativeDefaults(Element annotationScope) {
     }
   }
 
+  // Disable checking of contracts.
+  @Override
+  protected ContractsFromMethod createContractsFromMethod() {
+    return new ContractsFromMethod(this) {
+      @Override
+      public Set<ConditionalPostcondition> getConditionalPostconditions(
+          ExecutableElement methodElement) {
+        return emptySet();
+      }
+
+      @Override
+      public Set<Precondition> getPreconditions(ExecutableElement executableElement) {
+        return emptySet();
+      }
+
+      @Override
+      public Set<Postcondition> getPostconditions(ExecutableElement executableElement) {
+        return emptySet();
+      }
+    };
+  }
+
   @Override
   protected void addComputedTypeAnnotations(Tree tree, AnnotatedTypeMirror type, boolean iUseFlow) {
     super.addComputedTypeAnnotations(