[dart2js] Define type parameters of tearoffs as Object*/Object?

Change-Id: I5fa828b68e3dba398bac322a526af40616266ce5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/138102
Commit-Queue: Joshua Litt <joshualitt@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>

Author: joshualitt

pkg/compiler/lib/src/elements/types.dart

@@ -2117,4 +2117,22 @@ abstract class DartTypes {
   DartType getTypeVariableBound(TypeVariableEntity element);
 
   List<Variance> getTypeVariableVariances(ClassEntity cls);
+
+  DartType getTearOffParameterType(
+      DartType type, bool isCovariant, bool isNonNullableByDefaultLibrary) {
+    if (isCovariant) {
+      // A covariant parameter has type `Object` in the method signature.
+      var objectType = commonElements.objectType;
+      if (useNullSafety) {
+        if (isNonNullableByDefaultLibrary) {
+          return nullableType(objectType);
+        } else {
+          return legacyType(objectType);
+        }
+      } else {
+        return objectType;
+      }
+    }
+    return type;
+  }
 }

pkg/compiler/lib/src/js_model/element_map_impl.dart

@@ -870,11 +870,12 @@ class JsKernelToElementMap implements JsToElementMap, IrToElementMap {
     List<DartType> optionalParameterTypes = <DartType>[];
 
     DartType getParameterType(ir.VariableDeclaration variable) {
-      if (variable.isCovariant || variable.isGenericCovariantImpl) {
-        // A covariant parameter has type `Object` in the method signature.
-        return commonElements.objectType;
-      }
-      return getDartType(variable.type);
+      // isCovariant implies this FunctionNode is a class Procedure.
+      var isCovariant = variable.isCovariant || variable.isGenericCovariantImpl;
+      var isFromNonNullableByDefaultLibrary = isCovariant &&
+          (node.parent as ir.Procedure).enclosingLibrary.isNonNullableByDefault;
+      return types.getTearOffParameterType(getDartType(variable.type),
+          isCovariant, isFromNonNullableByDefaultLibrary);
     }
 
     for (ir.VariableDeclaration variable in node.positionalParameters) {

pkg/compiler/lib/src/kernel/element_map_impl.dart

@@ -493,11 +493,12 @@ class KernelToElementMapImpl implements KernelToElementMap, IrToElementMap {
     List<DartType> optionalParameterTypes = <DartType>[];
 
     DartType getParameterType(ir.VariableDeclaration variable) {
-      if (variable.isCovariant || variable.isGenericCovariantImpl) {
-        // A covariant parameter has type `Object` in the method signature.
-        return commonElements.objectType;
-      }
-      return getDartType(variable.type);
+      // isCovariant implies this FunctionNode is a class Procedure.
+      var isCovariant = variable.isCovariant || variable.isGenericCovariantImpl;
+      var isFromNonNullableByDefaultLibrary = isCovariant &&
+          (node.parent as ir.Procedure).enclosingLibrary.isNonNullableByDefault;
+      return types.getTearOffParameterType(getDartType(variable.type),
+          isCovariant, isFromNonNullableByDefaultLibrary);
     }
 
     for (ir.VariableDeclaration variable in node.positionalParameters) {