Add try-catch block around the WeakMap and WeakSet tests.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=129386297

Author: shicks

src/com/google/javascript/jscomp/js/es6/weakmap.js

@@ -27,14 +27,18 @@ $jscomp.polyfill('WeakMap', function(NativeWeakMap) {
    */
   function isConformant() {
     if (!NativeWeakMap || !Object.seal) return false;
-    var x = Object.seal({});
-    var y = Object.seal({});
-    var map = new /** @type {function(new: WeakMap, !Array)} */ (NativeWeakMap)(
-        [[x, 2], [y, 3]]);
-    if (map.get(x) != 2 || map.get(y) != 3) return false;
-    map.delete(x);
-    map.set(y, 4);
-    return !map.has(x) && map.get(y) == 4;
+    try {
+      var x = Object.seal({});
+      var y = Object.seal({});
+      var map = new /** @type {function(new: WeakMap, !Array)} */ (
+          NativeWeakMap)([[x, 2], [y, 3]]);
+      if (map.get(x) != 2 || map.get(y) != 3) return false;
+      map.delete(x);
+      map.set(y, 4);
+      return !map.has(x) && map.get(y) == 4;
+    } catch (err) { // This should hopefully never happen, but let's be safe.
+      return false;
+    }
   }
   if (isConformant()) return NativeWeakMap;
 

src/com/google/javascript/jscomp/js/es6/weakset.js

@@ -14,7 +14,10 @@
  * limitations under the License.
  */
 
-'require es6/symbol es6/util/makeiterator util/polyfill es6/weakmap';
+'require es6/symbol';
+'require es6/util/makeiterator';
+'require util/polyfill';
+'require es6/weakmap';
 
 $jscomp.polyfill('WeakSet', function(NativeWeakSet) {
   /**
@@ -23,14 +26,18 @@ $jscomp.polyfill('WeakSet', function(NativeWeakSet) {
    */
   function isConformant() {
     if (!NativeWeakSet || !Object.seal) return false;
-    var x = Object.seal({});
-    var y = Object.seal({});
-    var set = new /** @type {function(new: WeakSet, !Array)} */ (NativeWeakSet)(
-        [x]);
-    if (!set.has(x) || set.has(y)) return false;
-    set.delete(x);
-    set.add(y);
-    return !set.has(x) && set.has(y);
+    try {
+      var x = Object.seal({});
+      var y = Object.seal({});
+      var set = new /** @type {function(new: WeakSet, !Array)} */ (
+          NativeWeakSet)([x]);
+      if (!set.has(x) || set.has(y)) return false;
+      set.delete(x);
+      set.add(y);
+      return !set.has(x) && set.has(y);
+    } catch (err) { // This should hopefully never happen, but let's be safe.
+      return false;
+    }
   }
   if (isConformant()) return NativeWeakSet;