fix: ensure graphql key (starting with numeric) is valid

packages/gatsby/src/schema/__tests__/create-key.js

@@ -2,7 +2,7 @@ const createKey = require(`../create-key`)
 
 describe(`createKey`, () => {
   it(`leaves valid strings as is`, () => {
-    ;[`01234`, `validstring`, `_hello`, `_`].forEach(input => {
+    ;[`validstring`, `_hello`, `_`].forEach(input => {
       expect(createKey(input)).toBe(input)
     })
   })
@@ -13,6 +13,7 @@ describe(`createKey`, () => {
       [`~/path/to/some/module`, `_xpathxtoxsomexmodule`],
       [`/*`, `_x`],
       [`/*.js`, `_xxjs`],
+      [`01234`, `_01234`],
     ].forEach(([input, output]) => {
       expect(createKey(input)).toBe(output)
     })

packages/gatsby/src/schema/__tests__/infer-graphql-type-test.js

@@ -82,6 +82,7 @@ describe(`GraphQL type inferance`, () => {
       "with space": 1,
       "with-hyphen": 2,
       "with resolver": `1012-11-01`,
+      123: 42,
       aBoolean: true,
       externalUrl: `https://example.com/awesome.jpg`,
       domain: `pizza.com`,
@@ -102,6 +103,7 @@ describe(`GraphQL type inferance`, () => {
       anObjectArray: [{ anotherObjectArray: [{ baz: `quz` }] }],
       "with space": 3,
       "with-hyphen": 4,
+      123: 24,
       frontmatter: {
         date: `1984-10-12`,
         title: `The world of slash and adventure`,
@@ -235,6 +237,7 @@ describe(`GraphQL type inferance`, () => {
         with_space
         with_hyphen
         with_resolver(formatString:"DD.MM.YYYY")
+        _123
       `
     )
 
@@ -245,6 +248,8 @@ describe(`GraphQL type inferance`, () => {
     expect(result.data.listNode[1].with_space).toEqual(3)
     expect(result.data.listNode[1].with_hyphen).toEqual(4)
     expect(result.data.listNode[0].with_resolver).toEqual(`01.11.1012`)
+    expect(result.data.listNode[0]._123).toEqual(42)
+    expect(result.data.listNode[1]._123).toEqual(24)
   })
 
   describe(`Handles dates`, () => {

packages/gatsby/src/schema/create-key.js

@@ -21,5 +21,10 @@ module.exports = (key: string): string => {
     return replaced.replace(/_/g, (char, index) => (index === 0 ? `_` : `x`))
   }
 
+  // key is invalid (starts with numeric); normalize with leading underscore
+  if (replaced.match(/^[0-9]/)) {
+    return `_` + replaced
+  }
+
   return replaced
 }