Handle switch statement in simple types inferrer.

tests/compiler/dart2js/mock_compiler.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.
 
 library mock_compiler;
 
 import "package:expect/expect.dart";
 import 'dart:collection';
 import 'dart:uri';
 
@@ skipping 79 matching lines @@
     operator[]=(index, value) {}
     var add;
   }
   class JSMutableArray extends JSArray implements JSMutableIndexable {}
   class JSFixedArray extends JSMutableArray {}
   class JSExtendableArray extends JSMutableArray {}
   class JSString extends Interceptor implements String, JSIndexable {
     var length;
     operator[](index) {}
     toString() {}
+    operator+(other) => this;
   }
   class JSNumber extends Interceptor implements num {
     // All these methods return a number to please type inferencing.
     operator-() => (this is JSInt) ? 42 : 42.0;
     operator +(other) => (this is JSInt) ? 42 : 42.0;
     operator -(other) => (this is JSInt) ? 42 : 42.0;
     operator ~/(other) => 42;
     operator /(other) => (this is JSInt) ? 42 : 42.0;
     operator *(other) => (this is JSInt) ? 42 : 42.0;
     operator %(other) => (this is JSInt) ? 42 : 42.0;
@@ skipping 298 matching lines @@
   }
 }
 
 class MockDeferredLoadTask extends DeferredLoadTask {
   MockDeferredLoadTask(Compiler compiler) : super(compiler);
 
   void registerMainApp(LibraryElement mainApp) {
     // Do nothing.
   }
 }