[lib] Rewrite Omit make it semi-homomorphic

Summary:
We should rewrite it so that optionality and variance can be preserved.

Changelog: [libdef] We rewrite the `Omit` type implementation. Now it will preserve the optionality and the variance of the input object. [example](https://flow.org/try/#1N4Igxg9gdgZglgcxALlAJwKYEMwBcD6aArlLnALYYrgA2WAzvXGCADQgYAeOBARgJ74AJhhhYiNXClzEM7DFCLl602e0hQhcMtCw18ufgAcqyMTXpyQ5LEZNCDx0+cvzOJtBQW49AOkMm+AhEWGhCKmZ6lgC+6hAi1AEYAAQA6toAFgDyRjpQeskAvMnAADpQyckwEBAA-MjJ9DJwUAgA3OWVvKENiuS8GGgdUNHl5RpNyRANWeTaADzpuNm5cLo0rMmlIN1o2wB8RSXRbckA9GfJg2gQaMkD1ZibUBDJNNAIg1doN2j05WwQAA3QZMaDUGxNQYgaJAA)

Reviewed By: jbrown215

Differential Revision: D47165009

fbshipit-source-id: 55d5558bb8f6a25e83829bca74c3e7a256bd424c

lib/core.js

@@ -2728,7 +2728,7 @@ type Pick<O: {...}, Keys: $Keys<O>> = {[key in Keys]: O[key]};
 /**
  * Omit specific fields from an object, e.g. Omit<O, 'foo' | 'bar'>
  */
-type Omit<O: {...}, Keys: $Keys<O>> = {[key in Exclude<$Keys<O>, Keys>]: O[key]};
+type Omit<O: {...}, Keys: $Keys<O>> = Pick<O, Exclude<$Keys<O>, Keys>>;
 
 /**
  * Construct an object type using string literals as keys with the given type,

tests/mapped_type_utilities/mapped_type_utilities.exp

@@ -1,6 +1,6 @@
 Error ------------------------------------------------------------------------------------------------------ omit.js:7:7
 
-Cannot get `noFoo.foo` because property `foo` is missing in `Omit` [1]. [prop-missing]
+Cannot get `noFoo.foo` because property `foo` is missing in `Pick` [1]. [prop-missing]
 
    omit.js:7:7
    7| noFoo.foo; // ERROR
@@ -14,7 +14,7 @@ References:
 
 Error ---------------------------------------------------------------------------------------------------- omit.js:12:12
 
-Cannot get `noFooOrBar.foo` because property `foo` is missing in `Omit` [1]. [prop-missing]
+Cannot get `noFooOrBar.foo` because property `foo` is missing in `Pick` [1]. [prop-missing]
 
    omit.js:12:12
    12| noFooOrBar.foo; // ERROR
@@ -28,7 +28,7 @@ References:
 
 Error ---------------------------------------------------------------------------------------------------- omit.js:13:12
 
-Cannot get `noFooOrBar.bar` because property `bar` (did you mean `baz`?) is missing in `Omit` [1]. [prop-missing]
+Cannot get `noFooOrBar.bar` because property `bar` (did you mean `baz`?) is missing in `Pick` [1]. [prop-missing]
 
    omit.js:13:12
    13| noFooOrBar.bar; // ERROR

tests/mapped_type_utilities/omit.js

@@ -13,3 +13,5 @@ noFooOrBar.foo; // ERROR
 noFooOrBar.bar; // ERROR
 noFooOrBar.baz; // OK
 (noFooOrBar:{baz: number}); // OK
+
+const o: Omit<{ foo?: string, bar: number; }, "bar"> = {}; // OK