Discard setters after calling them with constant values.

Closes #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;
 }