Add failure messages to gin and js mojom encoder

mojo/public/js/codec.js

Index: mojo/public/js/codec.js
diff --git a/mojo/public/js/codec.js b/mojo/public/js/codec.js
index ea4273a32ffd67e1398a1970a67ed1848f402e3f..5440abed92d3651e684a492f97673bee286d9d68 100644
--- a/mojo/public/js/codec.js
+++ b/mojo/public/js/codec.js
@@ -8,6 +8,7 @@ define("mojo/public/js/codec", [
 ], function(unicode, buffer) {
 
   var kErrorUnsigned = "Passing negative value to unsigned";
+  var kErrorArray = "Passing non-array-like to array";
 
   // Memory -------------------------------------------------------------------
 
@@ -352,6 +353,9 @@ define("mojo/public/js/codec", [
       this.encodePointer(val);
       return;
     }
+    if (val.length === undefined) {
+      throw new Error(kErrorArray);

[hansmuller, 2014/12/15 16:56:17]

I suppose another indication that val isn't an array (if val.length was defined)
would be that val.length < 0. But shouldn't you be using |val instanceof Array|
here?

+    }
     var numberOfElements = val.length;
     var encodedSize = kArrayHeaderSize + ((cls === PackedBool) ?
         Math.ceil(numberOfElements / 8) : cls.encodedSize * numberOfElements);