Fix #6500 (#10728)

src/lib/es2015.collection.d.ts

@@ -16,7 +16,6 @@ interface MapConstructor {
 declare var Map: MapConstructor;
 
 interface WeakMap<K, V> {
-    clear(): void;
     delete(key: K): boolean;
     get(key: K): V | undefined;
     has(key: K): boolean;
@@ -48,7 +47,6 @@ declare var Set: SetConstructor;
 
 interface WeakSet<T> {
     add(value: T): this;
-    clear(): void;
     delete(value: T): boolean;
     has(value: T): boolean;
 }