Contextually type object spread expressions

so `x = { ... { a: "a" } }` will be equivalent to `{ a: "a" }`.

Author: sandersn

src/compiler/checker.ts

@@ -12728,6 +12728,8 @@ namespace ts {
                 case SyntaxKind.PropertyAssignment:
                 case SyntaxKind.ShorthandPropertyAssignment:
                     return getContextualTypeForObjectLiteralElement(<ObjectLiteralElementLike>parent);
+                case SyntaxKind.SpreadAssignment:
+                    return getApparentTypeOfContextualType(parent.parent as ObjectLiteralExpression);
                 case SyntaxKind.ArrayLiteralExpression:
                     return getContextualTypeForElementExpression(node);
                 case SyntaxKind.ConditionalExpression: