Small inferrer improvments, and one refactoring in the inferrer.

tests/compiler/dart2js/simple_inferrer_try_catch_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 117 matching lines @@
 }
 
 returnDyn6() {
   try {
     throw 42;
   } catch (e) {
     return e;
   }
 }
 
+returnInt7() {
+  var a = 'foo';
+  try {
+    a = 42;
+    return a;
+  } catch (e) {
+  }
+  return 2;
+}
 
 main() {
   returnInt1();
   returnDyn1();
   returnInt2();
   returnDyn2();
   returnInt3();
   returnDyn3();
   returnInt4();
   returnDyn4();
   returnInt5();
   returnDyn5();
   returnInt6();
   returnDyn6();
+  returnInt7();
 }
 """;
 
 
 void main() {
   Uri uri = new Uri(scheme: 'source');
   var compiler = compilerFor(TEST, uri);
   compiler.runCompiler(uri);
   var typesTask = compiler.typesTask;
   var typesInferrer = typesTask.typesInferrer;
 
   checkReturn(String name, type) {
     var element = findElement(compiler, name);
     Expect.equals(type,
         typesInferrer.getReturnTypeOfElement(element).simplify(compiler));
   }
 
   checkReturn('returnInt1', typesTask.intType);
   checkReturn('returnInt2', typesTask.intType);
   checkReturn('returnInt3', typesTask.intType);
   checkReturn('returnInt4', typesTask.intType);
   checkReturn('returnInt5', typesTask.intType);
   checkReturn('returnInt6',
       new TypeMask.nonNullSubtype(compiler.intClass.rawType));
+  checkReturn('returnInt7', typesTask.intType);
 
   var subclassOfInterceptor =
       findTypeMask(compiler, 'Interceptor', 'nonNullSubclass');
 
   checkReturn('returnDyn1', subclassOfInterceptor);
   checkReturn('returnDyn2', subclassOfInterceptor);
   checkReturn('returnDyn3', subclassOfInterceptor);
   checkReturn('returnDyn4', subclassOfInterceptor);
   checkReturn('returnDyn5', subclassOfInterceptor);
   checkReturn('returnDyn6', typesTask.dynamicType);
 }