Make cache.identify return undefined instead of throwing.

Should help with `cache.identify`-related cases of issue #6673.

src/cache/inmemory/__tests__/entityStore.ts

@@ -1772,9 +1772,19 @@ describe('EntityStore', () => {
       c: 3,
     })).toBe('ABCs:{"b":2,"a":1,"c":3}');
 
-    expect(() => cache.identify(ABCs)).toThrowError(
-      "Missing field 'b' while computing key fields",
-    );
+    { // TODO Extact this to a helper function.
+      const consoleWarnSpy = jest.spyOn(console, "warn");
+      consoleWarnSpy.mockImplementation(() => {});
+      try {
+        expect(cache.identify(ABCs)).toBeUndefined();
+        expect(consoleWarnSpy).toHaveBeenCalledTimes(1);
+        expect(consoleWarnSpy).toHaveBeenCalledWith(
+          new Error("Missing field 'b' while computing key fields")
+        );
+      } finally {
+        consoleWarnSpy.mockRestore();
+      }
+    }
 
     expect(cache.readFragment({
       id: cache.identify({

src/cache/inmemory/inMemoryCache.ts

@@ -4,6 +4,7 @@ import './fixPolyfills';
 import { DocumentNode } from 'graphql';
 import { OptimisticWrapperFunction, wrap } from 'optimism';
 import { equal } from '@wry/equality';
+import { invariant } from 'ts-invariant';
 
 import { ApolloCache } from '../core/cache';
 import { Cache } from '../core/types/Cache';
@@ -334,8 +335,12 @@ export class InMemoryCache extends ApolloCache<NormalizedCacheObject> {
   // sure that none of the primary key fields have been renamed by aliasing.
   // If you pass a Reference object, its __ref ID string will be returned.
   public identify(object: StoreObject | Reference): string | undefined {
-    return isReference(object) ? object.__ref :
-      this.policies.identify(object)[0];
+    if (isReference(object)) return object.__ref;
+    try {
+      return this.policies.identify(object)[0];
+    } catch (e) {
+      invariant.warn(e);
+    }
   }
 
   public evict(options: Cache.EvictOptions): boolean {