Add nullability into signature

src/lib/es2015.collection.d.ts

@@ -2,7 +2,7 @@ interface Map<K, V> {
     clear(): void;
     delete(key: K): boolean;
     forEach(callbackfn: (value: V, index: K, map: Map<K, V>) => void, thisArg?: any): void;
-    get(key: K): V;
+    get(key: K): V | undefined;
     has(key: K): boolean;
     set(key: K, value?: V): Map<K, V>;
     readonly size: number;
@@ -18,7 +18,7 @@ declare var Map: MapConstructor;
 interface WeakMap<K, V> {
     clear(): void;
     delete(key: K): boolean;
-    get(key: K): V;
+    get(key: K): V | undefined;
     has(key: K): boolean;
     set(key: K, value?: V): WeakMap<K, V>;
 

src/lib/es2015.core.d.ts

@@ -10,7 +10,7 @@ interface Array<T> {
       * @param thisArg If provided, it will be used as the this value for each invocation of 
       * predicate. If it is not provided, undefined is used instead.
       */
-    find(predicate: (value: T, index: number, obj: Array<T>) => boolean, thisArg?: any): T;
+    find(predicate: (value: T, index: number, obj: Array<T>) => boolean, thisArg?: any): T | undefined;
 
     /** 
       * Returns the index of the first element in the array where predicate is true, and undefined 
@@ -21,7 +21,7 @@ interface Array<T> {
       * @param thisArg If provided, it will be used as the this value for each invocation of 
       * predicate. If it is not provided, undefined is used instead.
       */
-    findIndex(predicate: (value: T) => boolean, thisArg?: any): number;
+    findIndex(predicate: (value: T) => boolean, thisArg?: any): number | undefined;
 
     /**
       * Returns the this object after filling the section identified by start and end with value
@@ -379,7 +379,7 @@ interface String {
       * If there is no element at that position, the result is undefined. 
       * If a valid UTF-16 surrogate pair does not begin at pos, the result is the code unit at pos.
       */
-    codePointAt(pos: number): number;
+    codePointAt(pos: number): number | undefined;
 
     /**
       * Returns true if searchString appears as a substring of the result of converting this 

src/lib/es2015.symbol.d.ts

@@ -30,7 +30,7 @@ interface SymbolConstructor {
       * Otherwise, returns a undefined.
       * @param sym Symbol to find the key for.
       */
-    keyFor(sym: symbol): string;
+    keyFor(sym: symbol): string | undefined;
 }
 
 declare var Symbol: SymbolConstructor;