Use webpack to write declaration files, gives better reporting (#588)

src/index.ts

@@ -1,6 +1,5 @@
 import * as _ from 'lodash'
 import * as path from 'path'
-import * as fs from 'fs'
 
 import { findCompiledModule, cache } from './cache'
 import * as helpers from './helpers'
@@ -9,7 +8,6 @@ import { PathPlugin } from './paths-plugin'
 import { CheckerPlugin as _CheckerPlugin } from './watch-mode'
 
 const loaderUtils = require('loader-utils')
-const mkdirp = require('mkdirp')
 
 function loader(text) {
 	try {
@@ -170,8 +168,7 @@ function transform(
 		}
 
 		if (emitResult.declaration) {
-			mkdirp.sync(path.dirname(emitResult.declaration.name))
-			fs.writeFileSync(emitResult.declaration.name, emitResult.declaration.text)
+			instance.compiledDeclarations.push(emitResult.declaration)
 		}
 
 		return {

src/instance.ts

@@ -23,6 +23,7 @@ export interface Instance {
 	id: number
 	babelImpl?: any
 	compiledFiles: { [key: string]: boolean }
+	compiledDeclarations: { name: string; text: string }[]
 	configFilePath: string
 	compilerConfig: TsConfig
 	loaderConfig: LoaderConfig
@@ -160,6 +161,7 @@ export function ensureInstance(
 		id: ++id,
 		babelImpl,
 		compiledFiles: {},
+		compiledDeclarations: [],
 		loaderConfig,
 		configFilePath,
 		compilerConfig,
@@ -385,6 +387,7 @@ function setupWatchRun(compiler, instanceName: string) {
 		const lastCompiled = instance.compiledFiles
 
 		instance.compiledFiles = {}
+		instance.compiledDeclarations = [];
 		instance.startTime = startTime
 
 		const set = new Set(Object.keys(times).map(toUnix))
@@ -479,6 +482,17 @@ function setupAfterCompile(compiler, instanceName, forkChecker = false) {
 			Array.prototype.push.apply(compilation.fileDependencies, files.map(path.normalize))
 		})
 
+		instance.compiledDeclarations.forEach(declaration => {
+			const assetPath = path.relative(
+				compilation.compiler.context,
+				declaration.name
+			)
+			compilation.assets[assetPath] = {
+				source: () => declaration.text,
+				size: () => declaration.text.length
+			}
+		})
+
 		const timeStart = +new Date()
 		const diag = () =>
 			instance.loaderConfig.transpileOnly