[front end] Unify methods for making a type nullable.

Previously, there were three methods with slightly different behaviors:

- `OperationsCfe.makeNullableInternal`, which simply added `?` to the
  type.

- `OperationsCfe.getNullableType`, which contained special logic for
  intersection types, and also maintained a cache to minimize the
  number of type objects that needed to be allocated.

- `InferenceVisitorBase.computeNullable`, which also coerced `Never?`
  to `Null`.

With this change, there is just one core implementation,
`OperationsCfe.makeNullableInternal`, with all of these behaviors, and
then `InferenceVisitorBase.computeNullable` is a convenience method
that calls it.

Change-Id: I7b6a442d0f3ba66e89d39c50cf240f021344e4aa
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/398420
Commit-Queue: Chloe Stefantsova <[email protected]>
Auto-Submit: Paul Berry <[email protected]>
Reviewed-by: Chloe Stefantsova <[email protected]>

pkg/front_end/lib/src/type_inference/inference_visitor.dart

@@ -6824,8 +6824,8 @@ class InferenceVisitorImpl extends InferenceVisitorBase
       instrumentation!.record(uriForInstrumentation, fileOffset, 'target',
           new InstrumentationValueForMember(equalsTarget.member!));
     }
-    DartType rightType =
-        operations.getNullableType(equalsTarget.getBinaryOperandType(this));
+    DartType rightType = operations
+        .makeNullableInternal(equalsTarget.getBinaryOperandType(this));
     DartType contextType =
         rightType.withDeclaredNullability(Nullability.nullable);
     rightResult = ensureAssignableResult(contextType, rightResult,

pkg/front_end/lib/src/type_inference/inference_visitor_base.dart

@@ -194,12 +194,8 @@ abstract class InferenceVisitorBase implements InferenceVisitor {
     return greatestClosure(type, topType: coreTypes.objectNullableRawType);
   }
 
-  DartType computeNullable(DartType type) {
-    if (type is NullType || type is NeverType) {
-      return const NullType();
-    }
-    return cfeOperations.getNullableType(type);
-  }
+  DartType computeNullable(DartType type) =>
+      cfeOperations.makeNullableInternal(type);
 
   // Coverage-ignore(suite): Not run.
   Expression createReachabilityError(int fileOffset, Message errorMessage) {

pkg/front_end/lib/src/type_inference/type_inference_engine.dart

@@ -636,22 +636,6 @@ class OperationsCfe
     return new SharedTypeView(result);
   }
 
-  DartType getNullableType(DartType type) {
-    // Note that the [IntersectionType.withDeclaredNullability] is special so
-    // we don't trust it.
-    if (type.declaredNullability == Nullability.nullable &&
-        type is! IntersectionType) {
-      return type;
-    }
-    DartType? cached = typeCacheNullable[type];
-    if (cached != null) {
-      return cached;
-    }
-    DartType result = type.withDeclaredNullability(Nullability.nullable);
-    typeCacheNullable[type] = result;
-    return result;
-  }
-
   @override
   SharedTypeView<DartType> variableType(VariableDeclaration variable) {
     if (variable is VariableDeclarationImpl) {
@@ -763,7 +747,22 @@ class OperationsCfe
 
   @override
   DartType makeNullableInternal(DartType type) {
-    return type.withDeclaredNullability(Nullability.nullable);
+    if (type is NullType || type is NeverType) {
+      return const NullType();
+    }
+    // Note that the [IntersectionType.withDeclaredNullability] is special so
+    // we don't trust it.
+    if (type.declaredNullability == Nullability.nullable &&
+        type is! IntersectionType) {
+      return type;
+    }
+    DartType? cached = typeCacheNullable[type];
+    if (cached != null) {
+      return cached;
+    }
+    DartType result = type.withDeclaredNullability(Nullability.nullable);
+    typeCacheNullable[type] = result;
+    return result;
   }
 
   @override