Move promise polyfill into polyfill subdirectory. NFC (#15939)

This lays the groundwork to add more polyfills over time.

Since its preferable to use standard polyfills (not authored as part of
emscripten), we exclude this directory from lint checking (i.e.  we
treat it as third-party code).

Split out from #15822

Author: sbc100

.eslintrc.yml

@@ -11,6 +11,7 @@ ignorePatterns:
   - "cache/"
   - "third_party/"
   - "tests/"
+  - "src/polyfill/"
   - "src/library*.js"
   - "src/runtime_*.js"
   - "src/shell*.js"
@@ -30,7 +31,6 @@ ignorePatterns:
   - "src/memoryprofiler.js"
   - "src/support.js"
   - "src/gl-matrix.js"
-  - "src/promise_polyfill.js"
   - "src/headless.js"
   - "src/headlessCanvas.js"
   - "src/emscripten-source-map.min.js"

src/promise_polyfill.js renamed to src/polyfill/promise.js

@@ -40,7 +40,7 @@ var Module = typeof {{{ EXPORT_NAME }}} !== 'undefined' ? {{{ EXPORT_NAME }}} :
 // Include a Promise polyfill for legacy browsers. This is needed either for
 // wasm2js, where we polyfill the wasm API which needs Promises, or when using
 // modularize which creates a Promise for when the module is ready.
-#include "promise_polyfill.js"
+#include "polyfill/promise.js"
 #endif
 
 // See https://caniuse.com/mdn-javascript_builtins_object_assign