Introduce an ElementTypeMask to recognize simple constraints like "the type of parameter foo is the…

tests/compiler/dart2js/simple_inferrer_test.dart

 // Copyright (c) 2013, 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/types/types.dart'
     show TypeMask;
 
 import 'compiler_helper.dart';
@@ skipping 341 matching lines @@
 }
 
 testReturnElementOfConstList1() {
   return const [42][0];
 }
 
 testReturnElementOfConstList2() {
   return topLevelConstList[0];
 }
 
+testReturnItselfOrInt(a) {
+  if (a) return 42;
+  return testReturnItselfOrInt(a);
+}
+
 var topLevelConstList = const [42];
 
 get topLevelGetter => 42;
 returnDynamic() => topLevelGetter(42);
 returnTopLevelGetter() => topLevelGetter;
 
 class A {
   factory A() = A.generative;
   A.generative();
   operator==(other) => 42;
@@ skipping 88 matching lines @@
          ..returnInt2()
          ..returnInt3()
          ..returnInt4()
          ..returnInt5()
          ..returnInt6()
          ..returnInt7()
          ..returnInt8()
          ..returnInt9();
   testReturnElementOfConstList1();
   testReturnElementOfConstList2();
+  testReturnItselfOrInt(topLevelGetter());
 }
 """;
 
 void main() {
   Uri uri = new Uri(scheme: 'source');
   var compiler = compilerFor(TEST, uri);
   compiler.runCompiler(uri);
   var typesInferrer = compiler.typesTask.typesInferrer;
 
   checkReturn(String name, type) {
@@ skipping 54 matching lines @@
       .union(typesInferrer.doubleType, compiler).nullable().simplify(compiler));
   checkReturn('testSwitch2', typesInferrer.intType);
   checkReturn('testSwitch3', interceptorType.nullable());
   checkReturn('testSwitch4', typesInferrer.intType);
   checkReturn('testContinue1', interceptorType.nullable());
   checkReturn('testBreak1', interceptorType.nullable());
   checkReturn('testContinue2', interceptorType.nullable());
   checkReturn('testBreak2', typesInferrer.intType.nullable());
   checkReturn('testReturnElementOfConstList1', typesInferrer.intType);
   checkReturn('testReturnElementOfConstList2', typesInferrer.intType);
+  checkReturn('testReturnItselfOrInt', typesInferrer.intType);
 
   checkReturnInClass(String className, String methodName, type) {
     var cls = findElement(compiler, className);
     var element = cls.lookupLocalMember(buildSourceString(methodName));
     Expect.equals(type,
         typesInferrer.internal.returnTypeOf[element].simplify(compiler));
   }
 
   checkReturnInClass('A', 'returnInt1', typesInferrer.intType);
   checkReturnInClass('A', 'returnInt2', typesInferrer.intType);
@@ skipping 14 matching lines @@
   checkReturnInClass('B', 'returnInt9', typesInferrer.intType);
 
   checkFactoryConstructor(String className) {
     var cls = findElement(compiler, className);
     var element = cls.localLookup(buildSourceString(className));
     Expect.equals(new TypeMask.nonNullExact(cls.rawType),
                   typesInferrer.internal.returnTypeOf[element]);
   }
   checkFactoryConstructor('A');
 }