Add test for the decorated method beginning with underscore

tests/cases/compiler/decoratorWithUnderscoreMethod.ts

@@ -0,0 +1,18 @@
+// @noemithelpers: true
+// @experimentaldecorators: true
+
+declare var console : { log(arg: string): void };
+function dec(): Function {
+    return function (target: any, propKey: string, descr: PropertyDescriptor): void {
+        console.log(target[propKey]);
+        //logs undefined
+        //propKey has three underscores as prefix, but the method has only two underscores
+    };
+}
+
+class A {
+    @dec()
+    private __foo(bar: string): void {
+        // do something with bar
+    }
+}