dx: improve errors for read-only array mutations (#63)

Closes salesforce/lwc#1739.

src/read-only-handler.ts

@@ -1,4 +1,4 @@
-import { unwrap, isUndefined } from './shared';
+import { unwrap, isArray, isUndefined } from './shared';
 import { BaseProxyHandler, ReactiveMembraneShadowTarget } from './base-handler';
 
 const getterMap = new WeakMap<() => any, () => any>();
@@ -39,7 +39,10 @@ export class ReadOnlyHandler extends BaseProxyHandler {
     set(shadowTarget: ReactiveMembraneShadowTarget, key: PropertyKey, value: any): boolean {
         if (process.env.NODE_ENV !== 'production') {
             const { originalTarget } = this;
-            throw new Error(`Invalid mutation: Cannot set "${key.toString()}" on "${originalTarget}". "${originalTarget}" is read-only.`);
+            const msg = isArray(originalTarget) ?
+                `Invalid mutation: Cannot mutate array at index ${key.toString()}. Array is read-only.` :
+                `Invalid mutation: Cannot set "${key.toString()}" on "${originalTarget}". "${originalTarget}" is read-only.`;
+            throw new Error(msg);
         }
         return false;
     }

test/read-only-handler.spec.ts

@@ -425,4 +425,14 @@ describe('ReadOnlyHandler', () => {
             expect(wet.foo).toBe(1);
         });
     });
+    it('should throw a helpful message when attempting to mutate an array', function () {
+        const target = new ReactiveMembrane();
+        const obj = [];
+        const property = target.getReadOnlyProxy(obj);
+        expect(() => {
+            property.push("some object");
+        }).toThrowErrorMatchingInlineSnapshot(
+            `"Invalid mutation: Cannot mutate array at index 0. Array is read-only."`
+        );
+    });
 });