fix(forms): change Array.reduce usage to Array.forEach (#35349)

There is currently a bug in Chrome 80 that makes Array.reduce
not work according to spec. The functionality in forms that
retrieves controls from FormGroups and FormArrays (`form.get`)
relied on Array.reduce, so the Chrome bug broke forms for
many users.

This commit refactors our forms code to rely on Array.forEach
instead of Array.reduce to fix forms while we are waiting
for the Chrome fix to go live.

See https://bugs.chromium.org/p/chromium/issues/detail?id=1049982.

PR Close #35349

packages/forms/src/model.ts

@@ -52,17 +52,21 @@ function _find(control: AbstractControl, path: Array<string|number>| string, del
   }
   if (Array.isArray(path) && path.length === 0) return null;
 
-  return path.reduce((v: AbstractControl | null, name) => {
-    if (v instanceof FormGroup) {
-      return v.controls.hasOwnProperty(name as string) ? v.controls[name] : null;
+  // Not using Array.reduce here due to a Chrome 80 bug
+  // https://bugs.chromium.org/p/chromium/issues/detail?id=1049982
+  let controlToFind: AbstractControl|null = control;
+  path.forEach((name: string | number) => {
+    if (controlToFind instanceof FormGroup) {
+      controlToFind = controlToFind.controls.hasOwnProperty(name as string) ?
+          controlToFind.controls[name] :
+          null;
+    } else if (controlToFind instanceof FormArray) {
+      controlToFind = controlToFind.at(<number>name) || null;
+    } else {
+      controlToFind = null;
     }
-
-    if (v instanceof FormArray) {
-      return v.at(<number>name) || null;
-    }
-
-    return null;
-  }, control);
+  });
+  return controlToFind;
 }
 
 function coerceToValidator(

packages/forms/src/validators.ts

@@ -465,9 +465,13 @@ function _executeAsyncValidators(control: AbstractControl, validators: AsyncVali
 }
 
 function _mergeErrors(arrayOfErrors: ValidationErrors[]): ValidationErrors|null {
-  const res: {[key: string]: any} =
-      arrayOfErrors.reduce((res: ValidationErrors | null, errors: ValidationErrors | null) => {
-        return errors != null ? {...res !, ...errors} : res !;
-      }, {});
+  let res: {[key: string]: any} = {};
+
+  // Not using Array.reduce here due to a Chrome 80 bug
+  // https://bugs.chromium.org/p/chromium/issues/detail?id=1049982
+  arrayOfErrors.forEach((errors: ValidationErrors | null) => {
+    res = errors != null ? {...res !, ...errors} : res !;
+  });
+
   return Object.keys(res).length === 0 ? null : res;
 }