Introduce a UnionTypeMask, currently limited to 4 types.

tests/compiler/dart2js/cpa_inference_test.dart

 // Copyright (c) 2012, 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.
 
 import "package:expect/expect.dart";
 import '../../../sdk/lib/_internal/compiler/implementation/scanner/scannerlib.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/source_file.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/types/types.dart';
 
 import "parser_helper.dart";
@@ skipping 1237 matching lines @@
     Expect.equals(convert(singleton(cls).union(nullSingleton)),
                   new TypeMask.exact(cls.rawType));
   }
 
   Expect.equals(convert(singleton(a).union(singleton(b))),
                 new TypeMask.nonNullSubclass(a.rawType));
 
   Expect.equals(convert(singleton(a).union(singleton(b)).union(nullSingleton)),
                 new TypeMask.subclass(a.rawType));
 
-  Expect.equals(convert(singleton(b).union(singleton(d))),
-                new TypeMask.nonNullSubtype(a.rawType));
+  Expect.equals(
+      convert(singleton(b).union(singleton(d))).simplify(result.compiler),
+      new TypeMask.nonNullSubtype(a.rawType));
 }
 
 testSelectors() {
   final String source = r"""
       // ABC <--- A
       //       `- BC <--- B
       //               `- C
 
       class ABC {}
       class A extends ABC {}
@@ skipping 25 matching lines @@
   ClassElement b = findElement(result.compiler, 'B');
   ClassElement c = findElement(result.compiler, 'C');
   ClassElement xy = findElement(result.compiler, 'XY');
   ClassElement x = findElement(result.compiler, 'X');
   ClassElement y = findElement(result.compiler, 'Y');
   ClassElement z = findElement(result.compiler, 'Z');
 
   Selector foo = new Selector.call(buildSourceString("foo"), null, 0);
 
   Expect.equals(
-      inferredType(foo),
+      inferredType(foo).simplify(result.compiler),
       new TypeMask.nonNullSubclass(abc.rawType));
   Expect.equals(
       inferredType(new TypedSelector.subclass(x.rawType, foo)),
       new TypeMask.nonNullExact(b.rawType));
   Expect.equals(
       inferredType(new TypedSelector.subclass(y.rawType, foo)),
       new TypeMask.nonNullExact(c.rawType));
   Expect.equals(
       inferredType(new TypedSelector.subclass(z.rawType, foo)),
       new TypeMask.nonNullExact(a.rawType));
   Expect.equals(
-      inferredType(new TypedSelector.subclass(xy.rawType, foo)),
+      inferredType(new TypedSelector.subclass(
+          xy.rawType, foo)).simplify(result.compiler),
       new TypeMask.nonNullSubclass(bc.rawType));
 
   Selector bar = new Selector.call(buildSourceString("bar"), null, 0);
 
   Expect.isNull(inferredType(bar));
 }
 
 void main() {
   testDynamicBackDoor();
   testLiterals();
@@ skipping 38 matching lines @@
   testListWithCapacity();
   testEmptyList();
   testJsCall();
   testIsCheck();
   testSeenClasses();
   testGoodGuys();
   testIntDoubleNum();
   testConcreteTypeToTypeMask();
   testSelectors();
 }