Fix inference bugs found by sra@ by changing how we merge if/then/else locals.

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 215 matching lines @@
   var c = topLevelGetter();
   if (c != null && c is! bool && c is! int) {
     return 42;
   } else if (c is String) {
     return c;
   } else {
     return 68;
   }
 }
 
+testIf1(a) {
+  var c = null;
+  if (a) {
+    c = 10;
+  } else {
+  }
+  return c;
+}
+
+testIf2(a) {
+  var c = null;
+  if (a) {
+  } else {
+    c = 10;
+  }
+  return c;
+}
+
 returnAsString() {
   return topLevelGetter() as String;
 }
 
 returnIntAsNum() {
   return 0 as num;
 }
 
 typedef int Foo();
 
@@ skipping 234 matching lines @@
   testIsCheck11(topLevelGetter());
   testIsCheck12(topLevelGetter());
   testIsCheck13(topLevelGetter());
   testIsCheck14(topLevelGetter());
   testIsCheck15(topLevelGetter());
   testIsCheck16(topLevelGetter());
   testIsCheck17(topLevelGetter());
   testIsCheck18(topLevelGetter());
   testIsCheck19(topLevelGetter());
   testIsCheck20();
+  testIf1(topLevelGetter());
+  testIf2(topLevelGetter());
   returnAsString();
   returnIntAsNum();
   returnAsTypedef();
   returnTopLevelGetter();
   testDeadCode();
   testLabeledIf();
   testSwitch1();
   testSwitch2();
   testSwitch3();
   testSwitch4();
@@ skipping 77 matching lines @@
   checkReturn('testIsCheck11', intType);
   checkReturn('testIsCheck12', typesTask.dynamicType);
   checkReturn('testIsCheck13', intType);
   checkReturn('testIsCheck14', typesTask.dynamicType);
   checkReturn('testIsCheck15', intType);
   checkReturn('testIsCheck16', typesTask.dynamicType);
   checkReturn('testIsCheck17', intType);
   checkReturn('testIsCheck18', typesTask.dynamicType);
   checkReturn('testIsCheck19', typesTask.dynamicType);
   checkReturn('testIsCheck20', typesTask.dynamicType.nonNullable());
+  checkReturn('testIf1', typesTask.intType.nullable());
+  checkReturn('testIf2', typesTask.intType.nullable());
   checkReturn('returnAsString',
       new TypeMask.subtype(compiler.stringClass.computeType(compiler)));
   checkReturn('returnIntAsNum', typesTask.intType);
   checkReturn('returnAsTypedef', typesTask.functionType.nullable());
   checkReturn('returnTopLevelGetter', typesTask.intType);
   checkReturn('testDeadCode', typesTask.intType);
   checkReturn('testLabeledIf', typesTask.intType.nullable());
   checkReturn('testSwitch1', typesTask.intType
       .union(typesTask.doubleType, compiler).nullable().simplify(compiler));
   checkReturn('testSwitch2', typesTask.intType);
@@ skipping 40 matching lines @@
   checkReturnInClass('B', 'returnInt9', typesTask.intType);
 
   checkFactoryConstructor(String className, String factoryName) {
     var cls = findElement(compiler, className);
     var element = cls.localLookup(buildSourceString(factoryName));
     Expect.equals(new TypeMask.nonNullExact(cls.rawType),
                   typesInferrer.getReturnTypeOfElement(element));
   }
   checkFactoryConstructor('A', '');
 }