Discard setters after calling them with constant values.

Closes benjamn#134.

lib/runtime/entry.js

@@ -98,6 +98,7 @@ Ep.addGetters = function (getters, constant) {
  ! (name in this.getters)) {
  this.getters[name] = getter;
  getter.constant = constant;
+ getter.runCount = 0;
  }
  }
 };
@@ -291,12 +292,27 @@ function forEachSetter(entry, names, callback) {
  var keyCount = keys.length;
 
  for (var j = 0; j < keyCount; ++j) {
- callSetterIfNecessary(
- setters[keys[j]],
- name,
- getExportByName(entry, name),
- callback
- );
+ var key = keys[j];
+ var value = getExportByName(entry, name);
+
+ callSetterIfNecessary(setters[key], name, value, callback);
+
+ var getter = entry.getters[name];
+ if (typeof getter === "function" &&
+ // Sometimes a getter function will throw because it's called
+ // before the variable it's supposed to return has been
+ // initialized, so we need to know that the getter function has
+ // run to completion at least once.
+ getter.runCount > 0 &&
+ getter.constant) {
+ // If we happen to know that this getter function has run
+ // successfully, and will never return a different value, then we
+ // can forget the corresponding setter, because we've already
+ // reported that constant value. Note that we can't forget the
+ // getter, because we need to remember the original value in case
+ // anyone tampers with entry.exports[name].
+ delete setters[key];
+ }
  }
  }
 }
@@ -334,8 +350,11 @@ function makeUniqueKey() {
 }
 
 function runGetter(entry, name) {
+ var getter = entry.getters[name];
  try {
- return entry.getters[name]();
+ var result = getter();
+ ++getter.runCount;
+ return result;
  } catch (e) {}
  return GETTER_ERROR;
 }