Encode indices as unsigned LEB128 instead of signed

asm.js

@@ -963,7 +963,7 @@ export class Assembler {
             }
             func.body.push(...this.leb128(def_value));
         } else {
-            func.body.push(...this.leb128(value));
+            func.body.push(...this.uleb128(value));
         }
     }