Abstract over the type system.

pkg/analyzer/test/generated/type_system_test.dart

Index: pkg/analyzer/test/generated/type_system_test.dart
diff --git a/pkg/analyzer/test/generated/type_system_test.dart b/pkg/analyzer/test/generated/type_system_test.dart
index c24297f2860b05a1763d94921daa9076a28485f4..160a4e75e6e94ae317583b9a5f0abe4975a31898 100644
--- a/pkg/analyzer/test/generated/type_system_test.dart
+++ b/pkg/analyzer/test/generated/type_system_test.dart
@@ -10,6 +10,7 @@ import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/resolver.dart';
 import 'package:analyzer/src/generated/testing/element_factory.dart';
 import 'package:analyzer/src/generated/testing/test_type_provider.dart';
+import 'package:analyzer/src/generated/utilities_dart.dart';
 import 'package:unittest/unittest.dart';
 
 import '../reflective_tests.dart';
@@ -18,6 +19,8 @@ import '../utils.dart';
 main() {
   initializeTestEnvironment();
   runReflectiveTests(TypeSystemTest);
+  runReflectiveTests(StrongSubtypingTest);
+  runReflectiveTests(StrongAssignabilityTest);
 }
 
 @reflectiveTest
@@ -39,7 +42,7 @@ class TypeSystemTest {
 
   void setUp() {
     typeProvider = new TestTypeProvider();
-    typeSystem = new TypeSystemImpl(typeProvider);
+    typeSystem = new TypeSystemImpl();
     FunctionTypeAliasElementImpl typeAlias =
         ElementFactory.functionTypeAliasElement('A');
     typeAlias.parameters = [];
@@ -160,14 +163,21 @@ class TypeSystemTest {
     DartType typeParam = ElementFactory.typeParameterElement('T').type;
     DartType interfaceType = ElementFactory.classElement2('A', []).type;
     expect(
-        typeSystem.getLeastUpperBound(dynamicType, dynamicType), dynamicType);
-    expect(typeSystem.getLeastUpperBound(voidType, voidType), voidType);
-    expect(typeSystem.getLeastUpperBound(bottomType, bottomType), bottomType);
-    expect(typeSystem.getLeastUpperBound(typeParam, typeParam), typeParam);
-    expect(typeSystem.getLeastUpperBound(interfaceType, interfaceType),
+        typeSystem.getLeastUpperBound(typeProvider, dynamicType, dynamicType),
+        dynamicType);
+    expect(typeSystem.getLeastUpperBound(typeProvider, voidType, voidType),
+        voidType);
+    expect(typeSystem.getLeastUpperBound(typeProvider, bottomType, bottomType),
+        bottomType);
+    expect(typeSystem.getLeastUpperBound(typeProvider, typeParam, typeParam),
+        typeParam);
+    expect(
+        typeSystem.getLeastUpperBound(
+            typeProvider, interfaceType, interfaceType),
         interfaceType);
     expect(
-        typeSystem.getLeastUpperBound(simpleFunctionType, simpleFunctionType),
+        typeSystem.getLeastUpperBound(
+            typeProvider, simpleFunctionType, simpleFunctionType),
         simpleFunctionType);
   }
 
@@ -329,7 +339,9 @@ class TypeSystemTest {
     // List<int>
     //
     InterfaceType listOfIntType = listType.substitute4(<DartType>[intType]);
-    expect(typeSystem.getLeastUpperBound(listOfIntType, listOfIntType),
+    expect(
+        typeSystem.getLeastUpperBound(
+            typeProvider, listOfIntType, listOfIntType),
         listOfIntType);
   }
 
@@ -353,6 +365,565 @@ class TypeSystemTest {
 
   void _checkLeastUpperBound(
       DartType type1, DartType type2, DartType expectedResult) {
-    expect(typeSystem.getLeastUpperBound(type1, type2), expectedResult);
+    expect(typeSystem.getLeastUpperBound(typeProvider, type1, type2),
+        expectedResult);
+  }
+}
+
+class TypeBuilder {
+  static FunctionType functionType(
+      List<DartType> parameters, DartType returnType,
+      {List<DartType> optional, Map<String, DartType> named}) {
+    return ElementFactory
+        .functionElement8(parameters, returnType,
+            optional: optional, named: named)
+        .type;
+  }
+}
+
+@reflectiveTest
+class StrongSubtypingTest {
+  TypeProvider typeProvider;
+  TypeSystem typeSystem;
+
+  DartType get bottomType => typeProvider.bottomType;
+  InterfaceType get doubleType => typeProvider.doubleType;
+  DartType get dynamicType => typeProvider.dynamicType;
+  InterfaceType get functionType => typeProvider.functionType;
+  InterfaceType get intType => typeProvider.intType;
+  InterfaceType get listType => typeProvider.listType;
+  InterfaceType get numType => typeProvider.numType;
+  InterfaceType get objectType => typeProvider.objectType;
+  InterfaceType get stringType => typeProvider.stringType;
+  DartType get voidType => VoidTypeImpl.instance;
+
+  void setUp() {
+    typeProvider = new TestTypeProvider();
+    typeSystem = new StrongTypeSystemImpl();
+  }
+
+  void test_isSubtypeOf_dynamic_isTop() {
+    DartType interfaceType = ElementFactory.classElement2('A', []).type;
+    List<DartType> equivalents = <DartType>[dynamicType, objectType];
+    List<DartType> subtypes = <DartType>[
+      intType,
+      doubleType,
+      numType,
+      stringType,
+      functionType,
+      interfaceType,
+      bottomType
+    ];
+    _checkGroups(dynamicType, equivalents: equivalents, subtypes: subtypes);
+  }
+
+  void test_isSubtypeOf_bottom_isBottom() {
+    DartType interfaceType = ElementFactory.classElement2('A', []).type;
+    List<DartType> equivalents = <DartType>[bottomType];
+    List<DartType> supertypes = <DartType>[
+      dynamicType,
+      objectType,
+      intType,
+      doubleType,
+      numType,
+      stringType,
+      functionType,
+      interfaceType
+    ];
+    _checkGroups(bottomType, equivalents: equivalents, supertypes: supertypes);
+  }
+
+  void test_isSubtypeOf_int() {
+    List<DartType> equivalents = <DartType>[intType];
+    List<DartType> supertypes = <DartType>[numType];
+    List<DartType> unrelated = <DartType>[doubleType];
+    _checkGroups(intType,
+        equivalents: equivalents, supertypes: supertypes, unrelated: unrelated);
+  }
+
+  void test_isSubtypeOf_double() {
+    List<DartType> equivalents = <DartType>[doubleType];
+    List<DartType> supertypes = <DartType>[numType];
+    List<DartType> unrelated = <DartType>[intType];
+    _checkGroups(doubleType,
+        equivalents: equivalents, supertypes: supertypes, unrelated: unrelated);
+  }
+
+  void test_isSubtypeOf_num() {
+    List<DartType> equivalents = <DartType>[numType];
+    List<DartType> supertypes = <DartType>[];
+    List<DartType> unrelated = <DartType>[stringType];
+    List<DartType> subtypes = <DartType>[intType, doubleType];
+    _checkGroups(numType,
+        equivalents: equivalents,
+        supertypes: supertypes,
+        unrelated: unrelated,
+        subtypes: subtypes);
+  }
+
+  void test_isSubtypeOf_classes() {
+    ClassElement classTop = ElementFactory.classElement2("A");
+    ClassElement classLeft = ElementFactory.classElement("B", classTop.type);
+    ClassElement classRight = ElementFactory.classElement("C", classTop.type);
+    ClassElement classBottom = ElementFactory.classElement("D", classLeft.type)
+      ..interfaces = <InterfaceType>[classRight.type];
+    InterfaceType top = classTop.type;
+    InterfaceType left = classLeft.type;
+    InterfaceType right = classRight.type;
+    InterfaceType bottom = classBottom.type;
+
+    _checkLattice(top, left, right, bottom);
+  }
+
+  void test_isSubtypeOf_simple_function() {
+    FunctionType top =
+        TypeBuilder.functionType(<DartType>[intType], objectType);
+    FunctionType left = TypeBuilder.functionType(<DartType>[intType], intType);
+    FunctionType right =
+        TypeBuilder.functionType(<DartType>[objectType], objectType);
+    FunctionType bottom =
+        TypeBuilder.functionType(<DartType>[objectType], intType);
+
+    _checkLattice(top, left, right, bottom);
+  }
+
+  void test_isSubtypeOf_call_method() {
+    ClassElementImpl classBottom = ElementFactory.classElement2("Bottom");
+    MethodElement methodBottom =
+        ElementFactory.methodElement("call", objectType, <DartType>[intType]);
+    classBottom.methods = <MethodElement>[methodBottom];
+
+    DartType top = TypeBuilder.functionType(<DartType>[intType], objectType);
+    InterfaceType bottom = classBottom.type;
+
+    _checkIsStrictSubtypeOf(bottom, top);
+  }
+
+  void test_isSubtypeOf_fuzzy_arrows() {
+    FunctionType top =
+        TypeBuilder.functionType(<DartType>[dynamicType], objectType);
+    FunctionType left =
+        TypeBuilder.functionType(<DartType>[objectType], objectType);
+    FunctionType right =
+        TypeBuilder.functionType(<DartType>[dynamicType], bottomType);
+    FunctionType bottom =
+        TypeBuilder.functionType(<DartType>[objectType], bottomType);
+
+    _checkLattice(top, left, right, bottom);
+  }
+
+  void test_isSubtypeOf_void_functions() {
+    FunctionType top = TypeBuilder.functionType(<DartType>[intType], voidType);
+    FunctionType bottom =
+        TypeBuilder.functionType(<DartType>[objectType], intType);
+
+    _checkIsStrictSubtypeOf(bottom, top);
+  }
+
+  void test_isSubtypeOf_named_optional() {
+    DartType r = TypeBuilder.functionType(<DartType>[intType], intType);
+    DartType o = TypeBuilder.functionType(<DartType>[], intType,
+        optional: <DartType>[intType]);
+    DartType n = TypeBuilder.functionType(<DartType>[], intType,
+        named: <String, DartType>{'x': intType});
+    DartType rr =
+        TypeBuilder.functionType(<DartType>[intType, intType], intType);
+    DartType ro = TypeBuilder.functionType(<DartType>[intType], intType,
+        optional: <DartType>[intType]);
+    DartType rn = TypeBuilder.functionType(<DartType>[intType], intType,
+        named: <String, DartType>{'x': intType});
+    DartType oo = TypeBuilder.functionType(<DartType>[], intType,
+        optional: <DartType>[intType, intType]);
+    DartType nn = TypeBuilder.functionType(<DartType>[], intType,
+        named: <String, DartType>{'x': intType, 'y': intType});
+    DartType nnn = TypeBuilder.functionType(<DartType>[], intType,
+        named: <String, DartType>{'x': intType, 'y': intType, 'z': intType});
+
+    _checkGroups(r,
+        equivalents: [r],
+        subtypes: [o, ro, rn, oo],
+        unrelated: [n, rr, nn, nnn]);
+    _checkGroups(o,
+        equivalents: [o], subtypes: [oo], unrelated: [n, rr, ro, rn, nn, nnn]);
+    _checkGroups(n,
+        equivalents: [n],
+        subtypes: [nn, nnn],
+        unrelated: [r, o, rr, ro, rn, oo]);
+    _checkGroups(rr,
+        equivalents: [rr],
+        subtypes: [ro, oo],
+        unrelated: [r, o, n, rn, nn, nnn]);
+    _checkGroups(ro,
+        equivalents: [ro], subtypes: [oo], unrelated: [o, n, rn, nn, nnn]);
+    _checkGroups(rn,
+        equivalents: [rn],
+        subtypes: [],
+        unrelated: [o, n, rr, ro, oo, nn, nnn]);
+    _checkGroups(oo,
+        equivalents: [oo], subtypes: [], unrelated: [n, rn, nn, nnn]);
+    _checkGroups(nn,
+        equivalents: [nn], subtypes: [nnn], unrelated: [r, o, rr, ro, rn, oo]);
+    _checkGroups(nnn,
+        equivalents: [nnn], subtypes: [], unrelated: [r, o, rr, ro, rn, oo]);
+  }
+
+  void test_isSubtypeOf_generics() {
+    ClassElementImpl LClass = ElementFactory.classElement2('L', ["T"]);
+    InterfaceType LType = LClass.type;
+    ClassElementImpl MClass = ElementFactory.classElement2('M', ["T"]);
+    DartType typeParam = MClass.typeParameters[0].type;
+    InterfaceType superType = LType.substitute4(<DartType>[typeParam]);
+    MClass.interfaces = <InterfaceType>[superType];
+    InterfaceType MType = MClass.type;
+
+    InterfaceType top = LType.substitute4(<DartType>[dynamicType]);
+    InterfaceType left = MType.substitute4(<DartType>[dynamicType]);
+    InterfaceType right = LType.substitute4(<DartType>[intType]);
+    InterfaceType bottom = MType.substitute4(<DartType>[intType]);
+
+    _checkLattice(top, left, right, bottom);
+  }
+
+  void _checkLattice(
+      DartType top, DartType left, DartType right, DartType bottom) {
+    _checkGroups(top,
+        equivalents: <DartType>[top],
+        subtypes: <DartType>[left, right, bottom]);
+    _checkGroups(left,
+        equivalents: <DartType>[left],
+        subtypes: <DartType>[bottom],
+        unrelated: <DartType>[right],
+        supertypes: <DartType>[top]);
+    _checkGroups(right,
+        equivalents: <DartType>[right],
+        subtypes: <DartType>[bottom],
+        unrelated: <DartType>[left],
+        supertypes: <DartType>[top]);
+    _checkGroups(bottom,
+        equivalents: <DartType>[bottom],
+        supertypes: <DartType>[top, left, right]);
+  }
+
+  void _checkGroups(DartType t1,
+      {List<DartType> equivalents,
+      List<DartType> unrelated,
+      List<DartType> subtypes,
+      List<DartType> supertypes}) {
+    if (equivalents != null) {
+      for (DartType t2 in equivalents) {
+        _checkEquivalent(t1, t2);
+      }
+    }
+    if (unrelated != null) {
+      for (DartType t2 in unrelated) {
+        _checkUnrelated(t1, t2);
+      }
+    }
+    if (subtypes != null) {
+      for (DartType t2 in subtypes) {
+        _checkIsStrictSubtypeOf(t2, t1);
+      }
+    }
+    if (supertypes != null) {
+      for (DartType t2 in supertypes) {
+        _checkIsStrictSubtypeOf(t1, t2);
+      }
+    }
+  }
+
+  void _checkUnrelated(DartType type1, DartType type2) {
+    _checkIsNotSubtypeOf(type1, type2);
+    _checkIsNotSubtypeOf(type2, type1);
+  }
+
+  void _checkEquivalent(DartType type1, DartType type2) {
+    _checkIsSubtypeOf(type1, type2);
+    _checkIsSubtypeOf(type2, type1);
+  }
+
+  void _checkIsStrictSubtypeOf(DartType type1, DartType type2) {
+    _checkIsSubtypeOf(type1, type2);
+    _checkIsNotSubtypeOf(type2, type1);
+  }
+
+  void _checkIsSubtypeOf(DartType type1, DartType type2) {
+    expect(typeSystem.isSubtypeOf(type1, type2), true);
+  }
+
+  void _checkIsNotSubtypeOf(DartType type1, DartType type2) {
+    expect(typeSystem.isSubtypeOf(type1, type2), false);
+  }
+}
+
+@reflectiveTest
+class StrongAssignabilityTest {
+  TypeProvider typeProvider;
+  TypeSystem typeSystem;
+
+  DartType get bottomType => typeProvider.bottomType;
+  InterfaceType get doubleType => typeProvider.doubleType;
+  DartType get dynamicType => typeProvider.dynamicType;
+  InterfaceType get functionType => typeProvider.functionType;
+  InterfaceType get intType => typeProvider.intType;
+  InterfaceType get listType => typeProvider.listType;
+  InterfaceType get numType => typeProvider.numType;
+  InterfaceType get objectType => typeProvider.objectType;
+  InterfaceType get stringType => typeProvider.stringType;
+  DartType get voidType => VoidTypeImpl.instance;
+
+  void setUp() {
+    typeProvider = new TestTypeProvider();
+    typeSystem = new StrongTypeSystemImpl();
+  }
+
+  void test_isAssignableTo_dynamic_isTop() {
+    DartType interfaceType = ElementFactory.classElement2('A', []).type;
+    List<DartType> interassignable = <DartType>[
+      dynamicType,
+      objectType,
+      intType,
+      doubleType,
+      numType,
+      stringType,
+      interfaceType,
+      bottomType
+    ];
+    _checkGroups(dynamicType, interassignable: interassignable);
+  }
+
+  void test_isAssignableTo_bottom_isBottom() {
+    DartType interfaceType = ElementFactory.classElement2('A', []).type;
+    List<DartType> interassignable = <DartType>[
+      dynamicType,
+      objectType,
+      intType,
+      doubleType,
+      numType,
+      stringType,
+      interfaceType,
+      bottomType
+    ];
+
+    _checkGroups(bottomType, interassignable: interassignable);
+  }
+
+  void test_isAssignableTo_int() {
+    DartType interfaceType = ElementFactory.classElement2('A', []).type;
+    List<DartType> interassignable = <DartType>[
+      dynamicType,
+      objectType,
+      intType,
+      numType,
+      bottomType
+    ];
+    List<DartType> unrelated = <DartType>[
+      doubleType,
+      stringType,
+      interfaceType,
+    ];
+
+    _checkGroups(intType,
+        interassignable: interassignable, unrelated: unrelated);
+  }
+
+  void test_isAssignableTo_double() {
+    DartType interfaceType = ElementFactory.classElement2('A', []).type;
+    List<DartType> interassignable = <DartType>[
+      dynamicType,
+      objectType,
+      doubleType,
+      numType,
+      bottomType
+    ];
+    List<DartType> unrelated = <DartType>[intType, stringType, interfaceType,];
+
+    _checkGroups(doubleType,
+        interassignable: interassignable, unrelated: unrelated);
+  }
+
+  void test_isAssignableTo_num() {
+    DartType interfaceType = ElementFactory.classElement2('A', []).type;
+    List<DartType> interassignable = <DartType>[
+      dynamicType,
+      objectType,
+      numType,
+      intType,
+      doubleType,
+      bottomType
+    ];
+    List<DartType> unrelated = <DartType>[stringType, interfaceType,];
+
+    _checkGroups(numType,
+        interassignable: interassignable, unrelated: unrelated);
+  }
+
+  void test_isAssignableTo_classes() {
+    ClassElement classTop = ElementFactory.classElement2("A");
+    ClassElement classLeft = ElementFactory.classElement("B", classTop.type);
+    ClassElement classRight = ElementFactory.classElement("C", classTop.type);
+    ClassElement classBottom = ElementFactory.classElement("D", classLeft.type)
+      ..interfaces = <InterfaceType>[classRight.type];
+    InterfaceType top = classTop.type;
+    InterfaceType left = classLeft.type;
+    InterfaceType right = classRight.type;
+    InterfaceType bottom = classBottom.type;
+
+    _checkLattice(top, left, right, bottom);
+  }
+
+  void test_isAssignableTo_simple_function() {
+    FunctionType top =
+        TypeBuilder.functionType(<DartType>[intType], objectType);
+    FunctionType left = TypeBuilder.functionType(<DartType>[intType], intType);
+    FunctionType right =
+        TypeBuilder.functionType(<DartType>[objectType], objectType);
+    FunctionType bottom =
+        TypeBuilder.functionType(<DartType>[objectType], intType);
+
+    _checkCrossLattice(top, left, right, bottom);
+  }
+
+  void test_isAssignableTo_call_method() {
+    ClassElementImpl classBottom = ElementFactory.classElement2("B");
+    MethodElement methodBottom =
+        ElementFactory.methodElement("call", objectType, <DartType>[intType]);
+    classBottom.methods = <MethodElement>[methodBottom];
+
+    DartType top = TypeBuilder.functionType(<DartType>[intType], objectType);
+    InterfaceType bottom = classBottom.type;
+
+    _checkIsAssignableTo(top, bottom);
+  }
+
+  void test_isAssignableTo_fuzzy_arrows() {
+    FunctionType top =
+        TypeBuilder.functionType(<DartType>[dynamicType], objectType);
+    FunctionType left =
+        TypeBuilder.functionType(<DartType>[objectType], objectType);
+    FunctionType right =
+        TypeBuilder.functionType(<DartType>[dynamicType], bottomType);
+    FunctionType bottom =
+        TypeBuilder.functionType(<DartType>[objectType], bottomType);
+
+    _checkCrossLattice(top, left, right, bottom);
+  }
+
+  void test_isAssignableTo_void_functions() {
+    FunctionType top = TypeBuilder.functionType(<DartType>[intType], voidType);
+    FunctionType bottom =
+        TypeBuilder.functionType(<DartType>[objectType], intType);
+
+    _checkEquivalent(bottom, top);
+  }
+
+  void test_isAssignableTo_named_optional() {
+    DartType r = TypeBuilder.functionType(<DartType>[intType], intType);
+    DartType o = TypeBuilder.functionType(<DartType>[], intType,
+        optional: <DartType>[intType]);
+    DartType n = TypeBuilder.functionType(<DartType>[], intType,
+        named: <String, DartType>{'x': intType});
+    DartType rr =
+        TypeBuilder.functionType(<DartType>[intType, intType], intType);
+    DartType ro = TypeBuilder.functionType(<DartType>[intType], intType,
+        optional: <DartType>[intType]);
+    DartType rn = TypeBuilder.functionType(<DartType>[intType], intType,
+        named: <String, DartType>{'x': intType});
+    DartType oo = TypeBuilder.functionType(<DartType>[], intType,
+        optional: <DartType>[intType, intType]);
+    DartType nn = TypeBuilder.functionType(<DartType>[], intType,
+        named: <String, DartType>{'x': intType, 'y': intType});
+    DartType nnn = TypeBuilder.functionType(<DartType>[], intType,
+        named: <String, DartType>{'x': intType, 'y': intType, 'z': intType});
+
+    _checkGroups(r,
+        interassignable: [r, o, ro, rn, oo], unrelated: [n, rr, nn, nnn]);
+    _checkGroups(o,
+        interassignable: [o, oo], unrelated: [n, rr, ro, rn, nn, nnn]);
+    _checkGroups(n,
+        interassignable: [n, nn, nnn], unrelated: [r, o, rr, ro, rn, oo]);
+    _checkGroups(rr,
+        interassignable: [rr, ro, oo], unrelated: [r, o, n, rn, nn, nnn]);
+    _checkGroups(ro, interassignable: [ro, oo], unrelated: [o, n, rn, nn, nnn]);
+    _checkGroups(rn,
+        interassignable: [rn], unrelated: [o, n, rr, ro, oo, nn, nnn]);
+    _checkGroups(oo, interassignable: [oo], unrelated: [n, rn, nn, nnn]);
+    _checkGroups(nn,
+        interassignable: [nn, nnn], unrelated: [r, o, rr, ro, rn, oo]);
+    _checkGroups(nnn,
+        interassignable: [nnn], unrelated: [r, o, rr, ro, rn, oo]);
+  }
+
+  void test_isAssignableTo_generics() {
+    ClassElementImpl LClass = ElementFactory.classElement2('L', ["T"]);
+    InterfaceType LType = LClass.type;
+    ClassElementImpl MClass = ElementFactory.classElement2('M', ["T"]);
+    DartType typeParam = MClass.typeParameters[0].type;
+    InterfaceType superType = LType.substitute4(<DartType>[typeParam]);
+    MClass.interfaces = <InterfaceType>[superType];
+    InterfaceType MType = MClass.type;
+
+    InterfaceType top = LType.substitute4(<DartType>[dynamicType]);
+    InterfaceType left = MType.substitute4(<DartType>[dynamicType]);
+    InterfaceType right = LType.substitute4(<DartType>[intType]);
+    InterfaceType bottom = MType.substitute4(<DartType>[intType]);
+
+    _checkCrossLattice(top, left, right, bottom);
+  }
+
+  void _checkCrossLattice(
+      DartType top, DartType left, DartType right, DartType bottom) {
+    _checkGroups(top, interassignable: <DartType>[top, left, right, bottom]);
+    _checkGroups(left, interassignable: <DartType>[top, left, right, bottom]);
+    _checkGroups(right, interassignable: <DartType>[top, left, right, bottom]);
+    _checkGroups(bottom, interassignable: <DartType>[top, left, right, bottom]);
+  }
+
+  void _checkLattice(
+      DartType top, DartType left, DartType right, DartType bottom) {
+    _checkGroups(top, interassignable: <DartType>[top, left, right, bottom]);
+    _checkGroups(left,
+        interassignable: <DartType>[top, left, bottom],
+        unrelated: <DartType>[right]);
+    _checkGroups(right,
+        interassignable: <DartType>[top, right, bottom],
+        unrelated: <DartType>[left]);
+    _checkGroups(bottom, interassignable: <DartType>[top, left, right, bottom]);
+  }
+
+  void _checkGroups(DartType t1,
+      {List<DartType> interassignable, List<DartType> unrelated}) {
+    if (interassignable != null) {
+      for (DartType t2 in interassignable) {
+        _checkEquivalent(t1, t2);
+      }
+    }
+    if (unrelated != null) {
+      for (DartType t2 in unrelated) {
+        _checkUnrelated(t1, t2);
+      }
+    }
+  }
+
+  void _checkUnrelated(DartType type1, DartType type2) {
+    _checkIsNotAssignableTo(type1, type2);
+    _checkIsNotAssignableTo(type2, type1);
+  }
+
+  void _checkEquivalent(DartType type1, DartType type2) {
+    _checkIsAssignableTo(type1, type2);
+    _checkIsAssignableTo(type2, type1);
+  }
+
+  void _checkIsStrictAssignableTo(DartType type1, DartType type2) {
+    _checkIsAssignableTo(type1, type2);
+    _checkIsNotAssignableTo(type2, type1);
+  }
+
+  void _checkIsAssignableTo(DartType type1, DartType type2) {
+    expect(typeSystem.isAssignableTo(type1, type2), true);
+  }
+
+  void _checkIsNotAssignableTo(DartType type1, DartType type2) {
+    expect(typeSystem.isAssignableTo(type1, type2), false);
   }
 }