[sprout] Add a mutate utility function

compiler/packages/sprout/src/shared-runtime.ts

@@ -32,6 +32,14 @@ export const CONST_NUMBER2 = 2;
 export const CONST_TRUE = true;
 export const CONST_FALSE = false;
 
+let count: number = 0;
+export function mutate(arg: any): void {
+  // don't mutate primitive
+  if (typeof arg != null && typeof arg === "object") {
+    arg["wat" + count++] = "joe";
+  }
+}
+
 export function graphql(value: string): string {
   return value;
 }