chore(dirtychecking): remove hashCode references

As Victor explained, and as I've verified in the dart tree, hashCodes have a specified default
implemenation in Dart which is not exposed in JS. While it is possible to implement similar
functionality, it's not clear that there's any real benefit to it, so it is simply removed
for the time being.

At a later point when it becomes clear that toString needs to uniquely identify objects, then
we can look at implementing a suitable algorithm for generating unique identifiers.

Closes #41

Author: caitp

src/dirty_checking.js

@@ -199,9 +199,7 @@ export class ChangeRecord {
   }
 
   toString() {
-    // Where the heck is hashCode from?
-    var hashCode = 0;
-    return `${_MODE_NAMES[this._mode]}[${this.field}]{${hashCode}}`;
+    return `${_MODE_NAMES[this._mode]}[${this.field}]`;
   }
 }