Declare SymbolConstructor for older versions of ES

Older versions of the ES standard do not define `Symbol#asyncIterator`.
This means our attempts to "manually" define `AsyncIterator` for
versions of ES that don't have it fail.

Following [1] and [2], our understanding is that the best existing
solution is to manually re-define `SymbolConstructor` to have this
member.

[1]: microsoft/TypeScript#13031
[2]: microsoft/TypeScript#8099

Author: hausdorff

CHANGELOG.md

@@ -1,3 +1,11 @@
+## v0.3.0
+
+### Improvements
+
+- Re-declare the global `SymbolConstructor` with `asyncIterator`, allowing users targeting ES
+  versions earlier than the 2018 to use this library.
+
+
 ## v0.2.0
 
 ### Improvements

nodejs/query/interfaces.ts

@@ -22,6 +22,12 @@
 
 ///////////////////////////////////////////////////////////////////////////////
 
+declare global {
+    export interface SymbolConstructor {
+        readonly asyncIterator: symbol;
+    }
+}
+
 export interface AsyncIterator<T> {
     next(value?: any): Promise<IteratorResult<T>>;
     return?(value?: any): Promise<IteratorResult<T>>;

nodejs/query/tsconfig.json

@@ -1,7 +1,7 @@
 {
     "compilerOptions": {
         "outDir": "bin",
-        "target": "es5",
+        "target": "es6",
         "module": "commonjs",
         "moduleResolution": "node",
         "declaration": true,
@@ -14,7 +14,6 @@
         "noImplicitReturns": true,
         "forceConsistentCasingInFileNames": true,
         "strictNullChecks": true,
-        "downlevelIteration": true,
     },
     "files": [
         "asyncQueryable.ts",