Added shortcut in checkAwaitedType for isolatedModules

src/compiler/checker.ts

@@ -9687,6 +9687,12 @@ namespace ts {
             // side of the `Promise` class, which would be `{ new <T>(...): Promise<T> }`.
             
             let promiseType = getTypeFromTypeNode(node.type);
+            if (promiseType === unknownType && compilerOptions.isolatedModules) {
+                // If we are compiling with isolatedModules, we may not be able to resolve the 
+                // type as a value. As such, we will just return unknownType;
+                return unknownType;
+            }
+            
             let promiseConstructor = getMergedSymbol(promiseType.symbol);
             if (!promiseConstructor || !symbolIsValue(promiseConstructor)) {
                 error(node, Diagnostics.Type_0_is_not_a_valid_async_function_return_type, typeToString(promiseType));