Handle huge unions better in createUnionOrIntersectionProperty

Author: weswigham

src/compiler/checker.ts

@@ -7769,7 +7769,7 @@ namespace ts {
         }
 
         function createUnionOrIntersectionProperty(containingType: UnionOrIntersectionType, name: __String): Symbol | undefined {
-            let props: Symbol[] | undefined;
+            const propSet = createMap<Symbol>();
             let indexTypes: Type[] | undefined;
             const isUnion = containingType.flags & TypeFlags.Union;
             const excludeModifiers = isUnion ? ModifierFlags.NonPublicAccessibilityModifier : 0;
@@ -7784,7 +7784,7 @@ namespace ts {
                     const modifiers = prop ? getDeclarationModifierFlagsFromSymbol(prop) : 0;
                     if (prop && !(modifiers & excludeModifiers)) {
                         commonFlags &= prop.flags;
-                        props = appendIfUnique(props, prop);
+                        propSet.set("" + getSymbolId(prop), prop);
                         checkFlags |= (isReadonlySymbol(prop) ? CheckFlags.Readonly : 0) |
                             (!(modifiers & ModifierFlags.NonPublicAccessibilityModifier) ? CheckFlags.ContainsPublic : 0) |
                             (modifiers & ModifierFlags.Protected ? CheckFlags.ContainsProtected : 0) |
@@ -7806,6 +7806,7 @@ namespace ts {
                     }
                 }
             }
+            const props = arrayFrom(propSet.values());
             if (!props) {
                 return undefined;
             }