fix(material-experimental/mdc-form-field): address feedback

Author: mmalerba

src/material-experimental/mdc-form-field/form-field.ts

@@ -211,9 +211,15 @@ export class MatFormField
   }
   set appearance(value: MatFormFieldAppearance) {
     const oldValue = this._appearance;
-    this._appearance = ['fill', 'outline'].includes(value)
-      ? value
-      : this._defaults?.appearance || DEFAULT_APPEARANCE;
+    const newAppearance = value || this._defaults?.appearance || DEFAULT_APPEARANCE;
+    if (typeof ngDevMode === 'undefined' || ngDevMode) {
+      if (newAppearance !== 'fill' && newAppearance !== 'outline') {
+        throw new Error(
+          `MatFormField: Invalid appearance "${newAppearance}", valid values are "fill" or "outline".`,
+        );
+      }
+    }
+    this._appearance = newAppearance;
     if (this._appearance === 'outline' && this._appearance !== oldValue) {
       this._refreshOutlineNotchWidth();