Fix issues uncovered by enabling Flow

Author: IvanGoncharov

src/validation/rules/KnownArgumentNames.js

@@ -15,17 +15,16 @@ import invariant from '../../jsutils/invariant';
 import suggestionList from '../../jsutils/suggestionList';
 import quotedOrList from '../../jsutils/quotedOrList';
 import * as Kind from '../../language/kinds';
-import type { GraphQLType } from '../../type/definition';
 
 
 export function unknownArgMessage(
   argName: string,
   fieldName: string,
-  type: GraphQLType,
+  typeName: string,
   suggestedArgs: Array<string>
 ): string {
   let message = `Unknown argument "${argName}" on field "${fieldName}" of ` +
-    `type "${String(type)}".`;
+    `type "${typeName}".`;
   if (suggestedArgs.length) {
     message += ` Did you mean ${quotedOrList(suggestedArgs)}?`;
   }

src/validation/rules/PossibleFragmentSpreads.js

@@ -12,6 +12,7 @@ import type { ValidationContext } from '../index';
 import { GraphQLError } from '../../error';
 import { doTypesOverlap } from '../../utilities/typeComparators';
 import { typeFromAST } from '../../utilities/typeFromAST';
+import { isCompositeType } from '../../type/definition';
 import type { GraphQLType } from '../../type/definition';
 
 
@@ -44,8 +45,8 @@ export function PossibleFragmentSpreads(context: ValidationContext): any {
     InlineFragment(node) {
       const fragType = context.getType();
       const parentType = context.getParentType();
-      if (fragType &&
-          parentType &&
+      if (isCompositeType(fragType) &&
+          isCompositeType(parentType) &&
           !doTypesOverlap(context.getSchema(), fragType, parentType)) {
         context.reportError(new GraphQLError(
           typeIncompatibleAnonSpreadMessage(parentType, fragType),