Revert "switch to core-js from es6-symbol and es6-shim"

This reverts commit 37b2e747d5.

lib/6to5/polyfill.js

@@ -1,16 +1,53 @@
 /* jshint newcap: false, freeze: false */
 
-require("core-js/shim");
-require("./transformation/transformers/es6-generators/runtime");
-
 var ensureSymbol = function (key) {
   Symbol[key] = Symbol[key] || Symbol(key);
 };
 
+var ensureProto = function (Constructor, key, val) {
+  var proto = Constructor.prototype;
+  if (!proto[key]) {
+    Object.defineProperty(proto, key, {
+      value: val
+    });
+  }
+};
+
+//
+
+if (typeof Symbol === "undefined") {
+  require("es6-symbol/implement");
+
+  var globSymbols = {};
+
+  Symbol.for = function (key) {
+    return globSymbols[key] = globSymbols[key] || Symbol(key);
+  };
+
+  Symbol.keyFor = function (sym) {
+    return sym.__description__;
+  };
+}
+
+require("es6-shim");
+require("./transformation/transformers/es6-generators/runtime");
+
 ensureSymbol("species");
-ensureSymbol("hasInstance");
-ensureSymbol("isConcatSpreadable");
-ensureSymbol("isRegExp");
-ensureSymbol("toPrimitive");
-ensureSymbol("toStringTag");
-ensureSymbol("unscopables");
+
+// Abstract references
+
+ensureSymbol("referenceGet");
+ensureSymbol("referenceSet");
+ensureSymbol("referenceDelete");
+
+ensureProto(Function, Symbol.referenceGet, function () { return this; });
+
+ensureProto(Map, Symbol.referenceGet, Map.prototype.get);
+ensureProto(Map, Symbol.referenceSet, Map.prototype.set);
+ensureProto(Map, Symbol.referenceDelete, Map.prototype.delete);
+
+if (global.WeakMap) {
+  ensureProto(WeakMap, Symbol.referenceGet, WeakMap.prototype.get);
+  ensureProto(WeakMap, Symbol.referenceSet, WeakMap.prototype.set);
+  ensureProto(WeakMap, Symbol.referenceDelete, WeakMap.prototype.delete);
+}