diff --git a/src/lib/es2015.core.d.ts b/src/lib/es2015.core.d.ts
index cfb300c784d74..ccada2e0069c9 100644
--- a/src/lib/es2015.core.d.ts
+++ b/src/lib/es2015.core.d.ts
@@ -298,6 +298,12 @@ interface ObjectConstructor {
      */
     getOwnPropertySymbols(o: any): symbol[];
 
+    /**
+      * Returns the names of the enumerable string properties and methods of an object.
+      * @param o Object that contains the properties and methods. This can be an object that you created or an existing Document Object Model (DOM) object.
+      */
+    keys(o: {}): string[];
+
     /**
      * Returns true if the values are the same value, false otherwise.
      * @param value1 The first value.
diff --git a/src/lib/es5.d.ts b/src/lib/es5.d.ts
index 458dc8b925216..acaf9351cc385 100644
--- a/src/lib/es5.d.ts
+++ b/src/lib/es5.d.ts
@@ -235,10 +235,10 @@ interface ObjectConstructor {
     isExtensible(o: any): boolean;
 
     /**
-      * Returns the names of the enumerable properties and methods of an object.
+      * Returns the names of the enumerable string properties and methods of an object.
       * @param o Object that contains the properties and methods. This can be an object that you created or an existing Document Object Model (DOM) object.
       */
-    keys(o: {}): string[];
+     keys(o: object): string[];
 }
 
 /**
diff --git a/tests/baselines/reference/conditionalTypeDoesntSpinForever.symbols b/tests/baselines/reference/conditionalTypeDoesntSpinForever.symbols
index d21db8eb90d5f..b5dcb78015b3f 100644
--- a/tests/baselines/reference/conditionalTypeDoesntSpinForever.symbols
+++ b/tests/baselines/reference/conditionalTypeDoesntSpinForever.symbols
@@ -403,9 +403,9 @@ export enum PubSubRecordIsStoredInRedisAsA {
       fields: () => new Set(Object.keys(soFar) as (keyof SO_FAR)[]),
 >fields : Symbol(fields, Decl(conditionalTypeDoesntSpinForever.ts, 94, 18))
 >Set : Symbol(Set, Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --))
->Object.keys : Symbol(ObjectConstructor.keys, Decl(lib.es5.d.ts, --, --))
+>Object.keys : Symbol(ObjectConstructor.keys, Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
 >Object : Symbol(Object, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
->keys : Symbol(ObjectConstructor.keys, Decl(lib.es5.d.ts, --, --))
+>keys : Symbol(ObjectConstructor.keys, Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
 >soFar : Symbol(soFar, Decl(conditionalTypeDoesntSpinForever.ts, 92, 29))
 >SO_FAR : Symbol(SO_FAR, Decl(conditionalTypeDoesntSpinForever.ts, 92, 21))
 
diff --git a/tests/baselines/reference/conditionalTypeDoesntSpinForever.types b/tests/baselines/reference/conditionalTypeDoesntSpinForever.types
index 1f4b4a62cd293..98a3c6ad08a4d 100644
--- a/tests/baselines/reference/conditionalTypeDoesntSpinForever.types
+++ b/tests/baselines/reference/conditionalTypeDoesntSpinForever.types
@@ -410,9 +410,9 @@ export enum PubSubRecordIsStoredInRedisAsA {
 >Set : SetConstructor
 >Object.keys(soFar) as (keyof SO_FAR)[] : (keyof SO_FAR)[]
 >Object.keys(soFar) : string[]
->Object.keys : (o: {}) => string[]
+>Object.keys : { (o: {}): string[]; (o: {}): string[]; }
 >Object : ObjectConstructor
->keys : (o: {}) => string[]
+>keys : { (o: {}): string[]; (o: {}): string[]; }
 >soFar : SO_FAR
 
       hasField: (fieldName: string | number | symbol) => fieldName in soFar
diff --git a/tests/baselines/reference/contextualExpressionTypecheckingDoesntBlowStack.symbols b/tests/baselines/reference/contextualExpressionTypecheckingDoesntBlowStack.symbols
index fc44c97996c92..60dfdd9983ae3 100644
--- a/tests/baselines/reference/contextualExpressionTypecheckingDoesntBlowStack.symbols
+++ b/tests/baselines/reference/contextualExpressionTypecheckingDoesntBlowStack.symbols
@@ -21,9 +21,9 @@ export default class Operation {
 
         for(const parameterLocation of Object.keys(parameterValues)) {
 >parameterLocation : Symbol(parameterLocation, Decl(contextualExpressionTypecheckingDoesntBlowStack.ts, 8, 17))
->Object.keys : Symbol(ObjectConstructor.keys, Decl(lib.es5.d.ts, --, --))
+>Object.keys : Symbol(ObjectConstructor.keys, Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
 >Object : Symbol(Object, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
->keys : Symbol(ObjectConstructor.keys, Decl(lib.es5.d.ts, --, --))
+>keys : Symbol(ObjectConstructor.keys, Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
 >parameterValues : Symbol(parameterValues, Decl(contextualExpressionTypecheckingDoesntBlowStack.ts, 6, 23))
 
             const parameter: any = (this as any).getParameter();;
diff --git a/tests/baselines/reference/contextualExpressionTypecheckingDoesntBlowStack.types b/tests/baselines/reference/contextualExpressionTypecheckingDoesntBlowStack.types
index 8d462f4dbd912..88feccd1a2f3a 100644
--- a/tests/baselines/reference/contextualExpressionTypecheckingDoesntBlowStack.types
+++ b/tests/baselines/reference/contextualExpressionTypecheckingDoesntBlowStack.types
@@ -21,9 +21,9 @@ export default class Operation {
         for(const parameterLocation of Object.keys(parameterValues)) {
 >parameterLocation : string
 >Object.keys(parameterValues) : string[]
->Object.keys : (o: {}) => string[]
+>Object.keys : { (o: {}): string[]; (o: {}): string[]; }
 >Object : ObjectConstructor
->keys : (o: {}) => string[]
+>keys : { (o: {}): string[]; (o: {}): string[]; }
 >parameterValues : any
 
             const parameter: any = (this as any).getParameter();;
diff --git a/tests/baselines/reference/inferFromGenericFunctionReturnTypes3.symbols b/tests/baselines/reference/inferFromGenericFunctionReturnTypes3.symbols
index 7281886e627e0..f89421cde07a7 100644
--- a/tests/baselines/reference/inferFromGenericFunctionReturnTypes3.symbols
+++ b/tests/baselines/reference/inferFromGenericFunctionReturnTypes3.symbols
@@ -204,9 +204,9 @@ function objectToMap(obj: any) {
 	return new Map(Object.keys(obj).map(key => [key, obj[key]]));
 >Map : Symbol(Map, Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --))
 >Object.keys(obj).map : Symbol(Array.map, Decl(lib.es5.d.ts, --, --))
->Object.keys : Symbol(ObjectConstructor.keys, Decl(lib.es5.d.ts, --, --))
+>Object.keys : Symbol(ObjectConstructor.keys, Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
 >Object : Symbol(Object, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
->keys : Symbol(ObjectConstructor.keys, Decl(lib.es5.d.ts, --, --))
+>keys : Symbol(ObjectConstructor.keys, Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
 >obj : Symbol(obj, Decl(inferFromGenericFunctionReturnTypes3.ts, 83, 21))
 >map : Symbol(Array.map, Decl(lib.es5.d.ts, --, --))
 >key : Symbol(key, Decl(inferFromGenericFunctionReturnTypes3.ts, 84, 37))
diff --git a/tests/baselines/reference/inferFromGenericFunctionReturnTypes3.types b/tests/baselines/reference/inferFromGenericFunctionReturnTypes3.types
index ccbeb99a11379..fa3cd2bcb89f6 100644
--- a/tests/baselines/reference/inferFromGenericFunctionReturnTypes3.types
+++ b/tests/baselines/reference/inferFromGenericFunctionReturnTypes3.types
@@ -231,9 +231,9 @@ function objectToMap(obj: any) {
 >Object.keys(obj).map(key => [key, obj[key]]) : [string, any][]
 >Object.keys(obj).map : <U>(callbackfn: (value: string, index: number, array: string[]) => U, thisArg?: any) => U[]
 >Object.keys(obj) : string[]
->Object.keys : (o: {}) => string[]
+>Object.keys : { (o: {}): string[]; (o: {}): string[]; }
 >Object : ObjectConstructor
->keys : (o: {}) => string[]
+>keys : { (o: {}): string[]; (o: {}): string[]; }
 >obj : any
 >map : <U>(callbackfn: (value: string, index: number, array: string[]) => U, thisArg?: any) => U[]
 >key => [key, obj[key]] : (key: string) => [string, any]
diff --git a/tests/baselines/reference/taggedTemplateStringsWithOverloadResolution3_ES6.errors.txt b/tests/baselines/reference/taggedTemplateStringsWithOverloadResolution3_ES6.errors.txt
index a4ba377af29fd..c5063689b51b4 100644
--- a/tests/baselines/reference/taggedTemplateStringsWithOverloadResolution3_ES6.errors.txt
+++ b/tests/baselines/reference/taggedTemplateStringsWithOverloadResolution3_ES6.errors.txt
@@ -89,7 +89,7 @@ tests/cases/conformance/es6/templates/taggedTemplateStringsWithOverloadResolutio
     fn5 `${ (n) => n.toFixed() }`; // will error; 'n' should have type 'string'.
                      ~~~~~~~
 !!! error TS2551: Property 'toFixed' does not exist on type 'string'. Did you mean 'fixed'?
-!!! related TS2728 /.ts/lib.es2015.core.d.ts:466:5: 'fixed' is declared here.
+!!! related TS2728 /.ts/lib.es2015.core.d.ts:472:5: 'fixed' is declared here.
     fn5 `${ (n) => n.substr(0) }`;
     
     
\ No newline at end of file