prevent incorrect codegen with empty path string

fixes googlearchive/polymer-expressions#42

investigation notes here: googlearchive/polymer-expressions#42 (comment)

it didn't look to me like isIndex was called on a hot path, so I just added a guard against empty string. Let me know if there's a smarter way to do that check.

[email protected]

Review URL: https://codereview.appspot.com/136950043

Author: John Messerly

src/observe.js

@@ -85,7 +85,7 @@
   var hasEval = detectEval();
 
   function isIndex(s) {
-    return +s === s >>> 0;
+    return +s === s >>> 0 && s !== '';
   }
 
   function toNumber(s) {

tests/test.js

@@ -156,6 +156,7 @@ suite('Path', function() {
     assertPath('foo["b\\"az"]', ['foo', 'b"az'], 'foo["b\\"az"]');
     assertPath("foo['b\\'az']", ['foo', "b'az"], 'foo["b\'az"]');
     assertPath(['a', 'b'], ['a', 'b'], 'a.b');
+    assertPath([''], [''], '[""]');
 
     function Foo(val) { this.val = val; }
     Foo.prototype.toString = function() { return 'Foo' + this.val; };