Allow resolveType to return a type name string

src/execution/__tests__/abstract-test.js

@@ -353,4 +353,79 @@ describe('Execute: Handles execution of abstract types', () => {
     });
   });
 
+  it('resolveType allows resolving with type name', async () => {
+    const PetType = new GraphQLInterfaceType({
+      name: 'Pet',
+      resolveType(obj) {
+        return obj instanceof Dog ? 'Dog' :
+          obj instanceof Cat ? 'Cat' :
+          null;
+      },
+      fields: {
+        name: { type: GraphQLString }
+      }
+    });
+
+    const DogType = new GraphQLObjectType({
+      name: 'Dog',
+      interfaces: [ PetType ],
+      fields: {
+        name: { type: GraphQLString },
+        woofs: { type: GraphQLBoolean },
+      }
+    });
+
+    const CatType = new GraphQLObjectType({
+      name: 'Cat',
+      interfaces: [ PetType ],
+      fields: {
+        name: { type: GraphQLString },
+        meows: { type: GraphQLBoolean },
+      }
+    });
+
+    const schema = new GraphQLSchema({
+      query: new GraphQLObjectType({
+        name: 'Query',
+        fields: {
+          pets: {
+            type: new GraphQLList(PetType),
+            resolve() {
+              return [
+                new Dog('Odie', true),
+                new Cat('Garfield', false)
+              ];
+            }
+          }
+        }
+      }),
+      types: [ CatType, DogType ]
+    });
+
+    const query = `{
+      pets {
+        name
+        ... on Dog {
+          woofs
+        }
+        ... on Cat {
+          meows
+        }
+      }
+    }`;
+
+    const result = await graphql(schema, query);
+
+    expect(result).to.jsonEqual({
+      data: {
+        pets: [
+          { name: 'Odie',
+            woofs: true },
+          { name: 'Garfield',
+            meows: false },
+        ]
+      }
+    });
+  });
+
 });

src/execution/execute.js

@@ -903,10 +903,15 @@ function completeAbstractValue(
   path: Array<string | number>,
   result: mixed
 ): mixed {
-  const runtimeType = returnType.resolveType ?
+  let runtimeType = returnType.resolveType ?
     returnType.resolveType(result, exeContext.contextValue, info) :
     defaultResolveTypeFn(result, exeContext.contextValue, info, returnType);
 
+  // If resolveType returns a string, we assume it's a GraphQLObjectType name.
+  if (typeof runtimeType === 'string') {
+    runtimeType = exeContext.schema.getType(runtimeType);
+  }
+
   if (!(runtimeType instanceof GraphQLObjectType)) {
     throw new GraphQLError(
       `Abstract type ${returnType.name} must resolve to an Object type at ` +

src/type/definition.js

@@ -471,7 +471,7 @@ export type GraphQLTypeResolveFn = (
   value: mixed,
   context: mixed,
   info: GraphQLResolveInfo
-) => ?GraphQLObjectType;
+) => ?GraphQLObjectType | ?string;
 
 export type GraphQLIsTypeOfFn = (
   source: mixed,