Disallow extending Map.

The one Google project that extended Map has fixed its code.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=139522732

src/com/google/javascript/jscomp/Es6ConvertSuperConstructorCalls.java

@@ -373,8 +373,7 @@ private boolean isUnextendableNativeClass(NodeTraversal t, String className) {
       case "Int32Array":
       case "Int8Array":
       case "InternalError":
-        // TODO(bradfordcsmith): Disallow Map again when client code is fixed.
-        // case "Map":
+      case "Map":
       case "Number":
       case "Promise":
       case "Proxy":