Abstract over the type system.

pkg/analyzer/lib/src/generated/error_verifier.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library engine.resolver.error_verifier;
 
 import 'dart:collection';
 import "dart:math" as math;
 
 import 'package:analyzer/src/generated/static_type_analyzer.dart';
@@ skipping 52 matching lines @@
    * The type representing the type 'int'.
    */
   InterfaceType _intType;
 
   /**
    * The object providing access to the types defined by the language.
    */
   final TypeProvider _typeProvider;
 
   /**
+   * The type system primitives
+   */
+  TypeSystem _typeSystem;
+
+  /**
    * The manager for the inheritance mappings.
    */
   final InheritanceManager _inheritanceManager;
 
   /**
    * A flag indicating whether the visitor is currently within a constructor
    * declaration that is 'const'.
    *
    * See [visitConstructorDeclaration].
    */
@@ skipping 174 matching lines @@
   /**
    * If `true`, mixins are allowed to inherit from types other than Object, and
    * are allowed to reference `super`.
    */
   final bool enableSuperMixins;
 
   /**
    * Initialize a newly created error verifier.
    */
   ErrorVerifier(this._errorReporter, this._currentLibrary, this._typeProvider,
+                this._typeSystem,
       this._inheritanceManager, this.enableSuperMixins) {
     this._isInSystemLibrary = _currentLibrary.source.isInSystemLibrary;
     this._hasExtUri = _currentLibrary.hasExtUri;
     _isEnclosingConstructorConst = false;
     _isInCatchClause = false;
     _isInStaticVariableDeclaration = false;
     _isInInstanceVariableDeclaration = false;
     _isInInstanceVariableInitializer = false;
     _isInConstructorInitializer = false;
     _isInStaticMethod = false;
@@ skipping 1070 matching lines @@
         _errorReporter.reportErrorForNode(
             StaticWarningCode.INVALID_OVERRIDE_NAMED, errorNameTarget, [
           overriddenParamName,
           overriddenExecutable.enclosingElement.displayName
         ]);
         return true;
       }
     }
     // SWC.INVALID_METHOD_OVERRIDE_RETURN_TYPE
     if (overriddenFTReturnType != VoidTypeImpl.instance &&
-        !overridingFTReturnType.isAssignableTo(overriddenFTReturnType)) {
+        !_typeSystem.isAssignableTo(overridingFTReturnType,overriddenFTReturnType)) {
       _errorReporter.reportTypeErrorForNode(!isGetter
               ? StaticWarningCode.INVALID_METHOD_OVERRIDE_RETURN_TYPE
               : StaticWarningCode.INVALID_GETTER_OVERRIDE_RETURN_TYPE,
           errorNameTarget, [
         overridingFTReturnType,
         overriddenFTReturnType,
         overriddenExecutable.enclosingElement.displayName
       ]);
       return true;
     }
     // SWC.INVALID_METHOD_OVERRIDE_NORMAL_PARAM_TYPE
     if (parameterLocations == null) {
       return false;
     }
     int parameterIndex = 0;
     for (int i = 0; i < overridingNormalPT.length; i++) {
-      if (!overridingNormalPT[i].isAssignableTo(overriddenNormalPT[i])) {
+      if (!_typeSystem.isAssignableTo(overridingNormalPT[i],overriddenNormalPT[i])) {
         _errorReporter.reportTypeErrorForNode(!isSetter
                 ? StaticWarningCode.INVALID_METHOD_OVERRIDE_NORMAL_PARAM_TYPE
                 : StaticWarningCode.INVALID_SETTER_OVERRIDE_NORMAL_PARAM_TYPE,
             parameterLocations[parameterIndex], [
           overridingNormalPT[i],
           overriddenNormalPT[i],
           overriddenExecutable.enclosingElement.displayName
         ]);
         return true;
       }
       parameterIndex++;
     }
     // SWC.INVALID_METHOD_OVERRIDE_OPTIONAL_PARAM_TYPE
     for (int i = 0; i < overriddenPositionalPT.length; i++) {
-      if (!overridingPositionalPT[i]
-          .isAssignableTo(overriddenPositionalPT[i])) {
+      if (!_typeSystem
+          .isAssignableTo(overridingPositionalPT[i], overriddenPositionalPT[i])) {
         _errorReporter.reportTypeErrorForNode(
             StaticWarningCode.INVALID_METHOD_OVERRIDE_OPTIONAL_PARAM_TYPE,
             parameterLocations[parameterIndex], [
           overridingPositionalPT[i],
           overriddenPositionalPT[i],
           overriddenExecutable.enclosingElement.displayName
         ]);
         return true;
       }
       parameterIndex++;
     }
     // SWC.INVALID_METHOD_OVERRIDE_NAMED_PARAM_TYPE &
     // SWC.INVALID_OVERRIDE_DIFFERENT_DEFAULT_VALUES
     for (String overriddenName in overriddenNamedPT.keys) {
       DartType overridingType = overridingNamedPT[overriddenName];
       if (overridingType == null) {
         // Error, this is never reached- INVALID_OVERRIDE_NAMED would have been
         // created above if this could be reached.
         continue;
       }
       DartType overriddenType = overriddenNamedPT[overriddenName];
-      if (!overriddenType.isAssignableTo(overridingType)) {
+      if (!_typeSystem.isAssignableTo(overriddenType,overridingType)) {
         // lookup the parameter for the error to select
         ParameterElement parameterToSelect = null;
         AstNode parameterLocationToSelect = null;
         for (int i = 0; i < parameters.length; i++) {
           ParameterElement parameter = parameters[i];
           if (parameter.parameterKind == ParameterKind.NAMED &&
               overriddenName == parameter.name) {
             parameterToSelect = parameter;
             parameterLocationToSelect = parameterLocations[i];
             break;
@@ skipping 306 matching lines @@
       }
       return false;
     }
     FunctionType redirectedType = redirectedElement.type;
     DartType redirectedReturnType = redirectedType.returnType;
     //
     // Report specific problem when return type is incompatible
     //
     FunctionType constructorType = declaration.element.type;
     DartType constructorReturnType = constructorType.returnType;
-    if (!redirectedReturnType.isAssignableTo(constructorReturnType)) {
+    if (!_typeSystem.isAssignableTo(redirectedReturnType,constructorReturnType)) {
       _errorReporter.reportErrorForNode(
           StaticWarningCode.REDIRECT_TO_INVALID_RETURN_TYPE,
           redirectedConstructor, [redirectedReturnType, constructorReturnType]);
       return true;
     }
     //
     // Check parameters
     //
-    if (!redirectedType.isSubtypeOf(constructorType)) {
+    if (!_typeSystem.isSubtypeOf(redirectedType,constructorType)) {
       _errorReporter.reportErrorForNode(
           StaticWarningCode.REDIRECT_TO_INVALID_FUNCTION_TYPE,
           redirectedConstructor, [redirectedType, constructorType]);
       return true;
     }
     return false;
   }
 
   /**
    * Check that the return [statement] of the form <i>return e;</i> is not in a
@@ skipping 25 matching lines @@
         return false;
       }
       _errorReporter.reportErrorForNode(
           CompileTimeErrorCode.RETURN_IN_GENERATIVE_CONSTRUCTOR,
           returnExpression);
       return true;
     }
     // RETURN_WITHOUT_VALUE
     if (returnExpression == null) {
       if (_inGenerator ||
-          _computeReturnTypeForMethod(null)
-              .isAssignableTo(expectedReturnType)) {
+          _typeSystem
+          .isAssignableTo(_computeReturnTypeForMethod(null), expectedReturnType)) {
         return false;
       }
       _hasReturnWithoutValue = true;
       _errorReporter.reportErrorForNode(
           StaticWarningCode.RETURN_WITHOUT_VALUE, statement);
       return true;
     } else if (_inGenerator) {
       // RETURN_IN_GENERATOR
       _errorReporter.reportErrorForNode(
           CompileTimeErrorCode.RETURN_IN_GENERATOR, statement);
@@ skipping 55 matching lines @@
    * [StaticWarningCode.MAP_KEY_TYPE_NOT_ASSIGNABLE], and
    * [StaticWarningCode.MAP_VALUE_TYPE_NOT_ASSIGNABLE].
    */
   bool _checkForArgumentTypeNotAssignable(Expression expression,
       DartType expectedStaticType, DartType actualStaticType,
       ErrorCode errorCode) {
     //
     // Warning case: test static type information
     //
     if (actualStaticType != null && expectedStaticType != null) {
-      if (!actualStaticType.isAssignableTo(expectedStaticType)) {
+      if (!_typeSystem.isAssignableTo(actualStaticType,expectedStaticType)) {
         _errorReporter.reportTypeErrorForNode(
             errorCode, expression, [actualStaticType, expectedStaticType]);
         return true;
       }
     }
     return false;
   }
 
   /**
    * Verify that the given [argument] can be assigned to its corresponding
@@ skipping 66 matching lines @@
    */
   bool _checkForAssignability(Expression expression, InterfaceType type,
       ErrorCode errorCode, List<Object> arguments) {
     if (expression == null) {
       return false;
     }
     DartType expressionType = expression.staticType;
     if (expressionType == null) {
       return false;
     }
-    if (expressionType.isAssignableTo(type)) {
+    if (_typeSystem.isAssignableTo(expressionType, type)) {
       return false;
     }
     _errorReporter.reportErrorForNode(errorCode, expression, arguments);
     return true;
   }
 
   /**
    * Verify that the given [expression] is not final.
    *
    * See [StaticWarningCode.ASSIGNMENT_TO_CONST],
@@ skipping 1237 matching lines @@
     // prepare expression type
     Expression expression = initializer.expression;
     if (expression == null) {
       return false;
     }
     // test the static type of the expression
     DartType staticType = getStaticType(expression);
     if (staticType == null) {
       return false;
     }
-    if (staticType.isAssignableTo(fieldType)) {
+    if (_typeSystem.isAssignableTo(staticType, fieldType)) {
       return false;
     }
     // report problem
     if (_isEnclosingConstructorConst) {
       // TODO(paulberry): this error should be based on the actual type of the
       // constant, not the static type.  See dartbug.com/21119.
       _errorReporter.reportTypeErrorForNode(
           CheckedModeCompileTimeErrorCode.CONST_FIELD_INITIALIZER_NOT_ASSIGNABLE,
           expression, [staticType, fieldType]);
     }
@@ skipping 127 matching lines @@
    * respectively.  If not, report the error using [returnType].
    */
   void _checkForIllegalReturnType(TypeName returnType) {
     if (returnType == null) {
       // No declared return type, so the return type must be dynamic, which is
       // assignable to everything.
       return;
     }
     if (_enclosingFunction.isAsynchronous) {
       if (_enclosingFunction.isGenerator) {
-        if (!_enclosingFunction.returnType
-            .isAssignableTo(_typeProvider.streamDynamicType)) {
+        if (!_typeSystem.isAssignableTo(_enclosingFunction.returnType,_typeProvider.streamDynamicType)) {
           _errorReporter.reportErrorForNode(
               StaticTypeWarningCode.ILLEGAL_ASYNC_GENERATOR_RETURN_TYPE,
               returnType);
         }
       } else {
-        if (!_enclosingFunction.returnType
-            .isAssignableTo(_typeProvider.futureDynamicType)) {
+        if (!_typeSystem.isAssignableTo(_enclosingFunction.returnType,_typeProvider.futureDynamicType)) {
           _errorReporter.reportErrorForNode(
               StaticTypeWarningCode.ILLEGAL_ASYNC_RETURN_TYPE, returnType);
         }
       }
     } else if (_enclosingFunction.isGenerator) {
-      if (!_enclosingFunction.returnType
-          .isAssignableTo(_typeProvider.iterableDynamicType)) {
+      if (!_typeSystem.isAssignableTo(_enclosingFunction.returnType,_typeProvider.iterableDynamicType)) {
         _errorReporter.reportErrorForNode(
             StaticTypeWarningCode.ILLEGAL_SYNC_GENERATOR_RETURN_TYPE,
             returnType);
       }
     }
   }
 
   /**
    * Verify that the given implements [clause] does not implement classes that
    * are deferred.
@@ skipping 352 matching lines @@
    */
   bool _checkForInvalidAssignment(Expression lhs, Expression rhs) {
     if (lhs == null || rhs == null) {
       return false;
     }
     VariableElement leftVariableElement = getVariableElement(lhs);
     DartType leftType = (leftVariableElement == null)
         ? getStaticType(lhs)
         : leftVariableElement.type;
     DartType staticRightType = getStaticType(rhs);
-    if (!staticRightType.isAssignableTo(leftType)) {
+    if (!_typeSystem.isAssignableTo(staticRightType,leftType)) {
       _errorReporter.reportTypeErrorForNode(
           StaticTypeWarningCode.INVALID_ASSIGNMENT, rhs, [
         staticRightType,
         leftType
       ]);
       return true;
     }
     return false;
   }
 
@@ skipping 14 matching lines @@
         ? getStaticType(lhs)
         : leftVariableElement.type;
     MethodElement invokedMethod = assignment.staticElement;
     if (invokedMethod == null) {
       return false;
     }
     DartType rightType = invokedMethod.type.returnType;
     if (leftType == null || rightType == null) {
       return false;
     }
-    if (!rightType.isAssignableTo(leftType)) {
+    if (!_typeSystem.isAssignableTo(rightType,leftType)) {
       _errorReporter.reportTypeErrorForNode(
           StaticTypeWarningCode.INVALID_ASSIGNMENT, rhs, [rightType, leftType]);
       return true;
     }
     return false;
   }
 
   /**
    * Check the given [initializer] to ensure that the field being initialized is
    * a valid field. The [fieldName] is the field name from the
@@ skipping 246 matching lines @@
       getterType = _getGetterType(propertyAccessorElement);
       setterType = _getSetterType(counterpartAccessor);
     } else if (propertyAccessorElement.isSetter) {
       setterType = _getSetterType(propertyAccessorElement);
       getterType = _getGetterType(counterpartAccessor);
     }
     // If either types are not assignable to each other, report an error
     // (if the getter is null, it is dynamic which is assignable to everything).
     if (setterType != null &&
         getterType != null &&
-        !getterType.isAssignableTo(setterType)) {
+        !_typeSystem.isAssignableTo(getterType,setterType)) {
       if (enclosingClassForCounterpart == null) {
         _errorReporter.reportTypeErrorForNode(
             StaticWarningCode.MISMATCHED_GETTER_AND_SETTER_TYPES,
             accessorDeclaration, [accessorTextName, setterType, getterType]);
         return true;
       } else {
         _errorReporter.reportTypeErrorForNode(
             StaticWarningCode.MISMATCHED_GETTER_AND_SETTER_TYPES_FROM_SUPERTYPE,
             accessorDeclaration, [
           accessorTextName,
@@ skipping 368 matching lines @@
         if ((elt is MethodElement && !elt.isAbstract) ||
             (elt is PropertyAccessorElement && !elt.isAbstract)) {
           // Since we are comparing two function types, we need to do the
           // appropriate type substitutions first ().
           FunctionType foundConcreteFT = _inheritanceManager
               .substituteTypeArgumentsInMemberFromInheritance(
                   concreteType, memberName, enclosingType);
           FunctionType requiredMemberFT = _inheritanceManager
               .substituteTypeArgumentsInMemberFromInheritance(
                   requiredMemberType, memberName, enclosingType);
-          if (foundConcreteFT.isSubtypeOf(requiredMemberFT)) {
+          if (_typeSystem.isSubtypeOf(foundConcreteFT,requiredMemberFT)) {
             continue;
           }
         }
       }
       // The not qualifying concrete executable element was found, add it to the
       // list.
       missingOverrides.add(executableElt);
     }
     // Now that we have the set of missing overrides, generate a warning on this
     // class.
@@ skipping 72 matching lines @@
   }
 
   /**
    * Check to ensure that the [condition] is of type bool, are. Otherwise an
    * error is reported on the expression.
    *
    * See [StaticTypeWarningCode.NON_BOOL_CONDITION].
    */
   bool _checkForNonBoolCondition(Expression condition) {
     DartType conditionType = getStaticType(condition);
-    if (conditionType != null && !conditionType.isAssignableTo(_boolType)) {
+    if (conditionType != null && !_typeSystem.isAssignableTo(conditionType,_boolType)) {
       _errorReporter.reportErrorForNode(
           StaticTypeWarningCode.NON_BOOL_CONDITION, condition);
       return true;
     }
     return false;
   }
 
   /**
    * Verify that the given assert [statement] has either a 'bool' or
    * '() -> bool' input.
    *
    * See [StaticTypeWarningCode.NON_BOOL_EXPRESSION].
    */
   bool _checkForNonBoolExpression(AssertStatement statement) {
     Expression expression = statement.condition;
     DartType type = getStaticType(expression);
     if (type is InterfaceType) {
-      if (!type.isAssignableTo(_boolType)) {
+      if (!_typeSystem.isAssignableTo(type,_boolType)) {
         _errorReporter.reportErrorForNode(
             StaticTypeWarningCode.NON_BOOL_EXPRESSION, expression);
         return true;
       }
     } else if (type is FunctionType) {
       FunctionType functionType = type;
       if (functionType.typeArguments.length == 0 &&
-          !functionType.returnType.isAssignableTo(_boolType)) {
+          !_typeSystem.isAssignableTo(functionType.returnType,_boolType)) {
         _errorReporter.reportErrorForNode(
             StaticTypeWarningCode.NON_BOOL_EXPRESSION, expression);
         return true;
       }
     }
     return false;
   }
 
   /**
    * Checks to ensure that the given [expression] is assignable to bool.
    *
    * See [StaticTypeWarningCode.NON_BOOL_NEGATION_EXPRESSION].
    */
   bool _checkForNonBoolNegationExpression(Expression expression) {
     DartType conditionType = getStaticType(expression);
-    if (conditionType != null && !conditionType.isAssignableTo(_boolType)) {
+    if (conditionType != null && !_typeSystem.isAssignableTo(conditionType,_boolType)) {
       _errorReporter.reportErrorForNode(
           StaticTypeWarningCode.NON_BOOL_NEGATION_EXPRESSION, expression);
       return true;
     }
     return false;
   }
 
   /**
    * Verify the given map [literal] either:
    * * has `const modifier`
@@ skipping 374 matching lines @@
         return false;
       }
       _errorReporter.reportTypeErrorForNode(
           StaticTypeWarningCode.RETURN_OF_INVALID_TYPE, returnExpression, [
         staticReturnType,
         expectedReturnType,
         _enclosingFunction.displayName
       ]);
       return true;
     }
-    if (staticReturnType.isAssignableTo(expectedReturnType)) {
+    if (_typeSystem.isAssignableTo(staticReturnType, expectedReturnType)) {
       return false;
     }
     _errorReporter.reportTypeErrorForNode(
         StaticTypeWarningCode.RETURN_OF_INVALID_TYPE, returnExpression, [
       staticReturnType,
       expectedReturnType,
       _enclosingFunction.displayName
     ]);
     return true;
     // TODO(brianwilkerson) Define a hint corresponding to the warning and
@@ skipping 57 matching lines @@
     NodeList<SwitchMember> members = statement.members;
     for (SwitchMember switchMember in members) {
       if (switchMember is! SwitchCase) {
         continue;
       }
       SwitchCase switchCase = switchMember as SwitchCase;
       // prepare 'case' type
       Expression caseExpression = switchCase.expression;
       DartType caseType = getStaticType(caseExpression);
       // check types
-      if (expressionType.isAssignableTo(caseType)) {
+      if (_typeSystem.isAssignableTo(expressionType, caseType)) {
         return false;
       }
       // report problem
       _errorReporter.reportErrorForNode(
           StaticWarningCode.SWITCH_EXPRESSION_NOT_ASSIGNABLE, expression, [
         expressionType,
         caseType
       ]);
       return true;
     }
@@ skipping 62 matching lines @@
     bool foundError = false;
     for (int i = 0; i < loopThroughIndex; i++) {
       TypeName argTypeName = typeNameArgList[i];
       DartType argType = argTypeName.type;
       DartType boundType = boundingElts[i].bound;
       if (argType != null && boundType != null) {
         if (typeArguments.length != 0 &&
             typeArguments.length == typeParameters.length) {
           boundType = boundType.substitute2(typeArguments, typeParameters);
         }
-        if (!argType.isSubtypeOf(boundType)) {
+        if (!_typeSystem.isSubtypeOf(argType,boundType)) {
           ErrorCode errorCode;
           if (_isInConstInstanceCreation) {
             errorCode = CompileTimeErrorCode.TYPE_ARGUMENT_NOT_MATCHING_BOUNDS;
           } else {
             errorCode = StaticTypeWarningCode.TYPE_ARGUMENT_NOT_MATCHING_BOUNDS;
           }
           _errorReporter.reportTypeErrorForNode(
               errorCode, argTypeName, [argType, boundType]);
           foundError = true;
         }
@@ skipping 166 matching lines @@
           if (fieldElement.isSynthetic) {
             _errorReporter.reportErrorForNode(
                 CompileTimeErrorCode.INITIALIZING_FORMAL_FOR_NON_EXISTENT_FIELD,
                 parameter, [parameter.identifier.name]);
           } else if (fieldElement.isStatic) {
             _errorReporter.reportErrorForNode(
                 CompileTimeErrorCode.INITIALIZING_FORMAL_FOR_STATIC_FIELD,
                 parameter, [parameter.identifier.name]);
           } else if (declaredType != null &&
               fieldType != null &&
-              !declaredType.isAssignableTo(fieldType)) {
+              !_typeSystem.isAssignableTo(declaredType,fieldType)) {
             _errorReporter.reportTypeErrorForNode(
                 StaticWarningCode.FIELD_INITIALIZING_FORMAL_NOT_ASSIGNABLE,
                 parameter, [declaredType, fieldType]);
           }
         } else {
           if (fieldElement.isSynthetic) {
             _errorReporter.reportErrorForNode(
                 CompileTimeErrorCode.INITIALIZING_FORMAL_FOR_NON_EXISTENT_FIELD,
                 parameter, [parameter.identifier.name]);
           } else if (fieldElement.isStatic) {
@@ skipping 135 matching lines @@
     DartType impliedReturnType;
     if (isYieldEach) {
       impliedReturnType = staticYieldedType;
     } else if (_enclosingFunction.isAsynchronous) {
       impliedReturnType =
           _typeProvider.streamType.substitute4(<DartType>[staticYieldedType]);
     } else {
       impliedReturnType =
           _typeProvider.iterableType.substitute4(<DartType>[staticYieldedType]);
     }
-    if (!impliedReturnType.isAssignableTo(declaredReturnType)) {
+    if (!_typeSystem.isAssignableTo(impliedReturnType,declaredReturnType)) {
       _errorReporter.reportTypeErrorForNode(
           StaticTypeWarningCode.YIELD_OF_INVALID_TYPE, yieldExpression, [
         impliedReturnType,
         declaredReturnType
       ]);
       return true;
     }
     if (isYieldEach) {
       // Since the declared return type might have been "dynamic", we need to
       // also check that the implied return type is assignable to generic
       // Stream/Iterable.
       DartType requiredReturnType;
       if (_enclosingFunction.isAsynchronous) {
         requiredReturnType = _typeProvider.streamDynamicType;
       } else {
         requiredReturnType = _typeProvider.iterableDynamicType;
       }
-      if (!impliedReturnType.isAssignableTo(requiredReturnType)) {
+      if (!_typeSystem.isAssignableTo(impliedReturnType,requiredReturnType)) {
         _errorReporter.reportTypeErrorForNode(
             StaticTypeWarningCode.YIELD_OF_INVALID_TYPE, yieldExpression, [
           impliedReturnType,
           requiredReturnType
         ]);
         return true;
       }
     }
     return false;
   }
 
   /**
    * Verify that if the given class [declaration] implements the class Function
    * that it has a concrete implementation of the call method.
    *
    * See [StaticWarningCode.FUNCTION_WITHOUT_CALL].
    */
   bool _checkImplementsFunctionWithoutCall(ClassDeclaration declaration) {
     if (declaration.isAbstract) {
       return false;
     }
     ClassElement classElement = declaration.element;
     if (classElement == null) {
       return false;
     }
-    if (!classElement.type.isSubtypeOf(_typeProvider.functionType)) {
+    if (!_typeSystem.isSubtypeOf(classElement.type,_typeProvider.functionType)) {
       return false;
     }
     // If there is a noSuchMethod method, then don't report the warning,
     // see dartbug.com/16078
     if (classElement.getMethod(FunctionElement.NO_SUCH_METHOD_METHOD_NAME) !=
         null) {
       return false;
     }
     ExecutableElement callMethod = _inheritanceManager.lookupMember(
         classElement, FunctionElement.CALL_METHOD_NAME);
@@ skipping 460 matching lines @@
     toCheck.add(type.element);
     // type arguments
     if (type is InterfaceType) {
       InterfaceType interfaceType = type;
       for (DartType typeArgument in interfaceType.typeArguments) {
         _addTypeToCheck(typeArgument);
       }
     }
   }
 }