diff --git a/packages/@angular/cli/models/webpack-config.ts b/packages/@angular/cli/models/webpack-config.ts
index 798a1a1208f7..9909ad91251d 100644
--- a/packages/@angular/cli/models/webpack-config.ts
+++ b/packages/@angular/cli/models/webpack-config.ts
@@ -1,5 +1,8 @@
+// @ignoreDep typescript - used only for type information
+import * as ts from 'typescript';
 import { AngularCompilerPlugin } from '@ngtools/webpack';
 import { readTsconfig } from '../utilities/read-tsconfig';
+import { requireProjectModule } from '../utilities/require-project-module';
 const webpackMerge = require('webpack-merge');
 import { CliConfig } from './config';
 import { BuildOptions } from './build-options';
@@ -20,6 +23,7 @@ export interface WebpackConfigOptions<T extends BuildOptions = BuildOptions> {
   buildOptions: T;
   appConfig: any;
   tsConfig: any;
+  supportES2015: boolean;
 }
 
 export class NgCliWebpackConfig<T extends BuildOptions = BuildOptions> {
@@ -39,7 +43,12 @@ export class NgCliWebpackConfig<T extends BuildOptions = BuildOptions> {
     const tsconfigPath = path.resolve(projectRoot, appConfig.root, appConfig.tsconfig);
     const tsConfig = readTsconfig(tsconfigPath);
 
-    this.wco = { projectRoot, buildOptions, appConfig, tsConfig };
+    const projectTs = requireProjectModule(projectRoot, 'typescript') as typeof ts;
+
+    const supportES2015 = tsConfig.options.target !== projectTs.ScriptTarget.ES3
+                        && tsConfig.options.target !== projectTs.ScriptTarget.ES5;
+
+    this.wco = { projectRoot, buildOptions, appConfig, tsConfig, supportES2015 };
   }
 
   public buildConfig() {
diff --git a/packages/@angular/cli/models/webpack-configs/browser.ts b/packages/@angular/cli/models/webpack-configs/browser.ts
index 5be79694ac31..973ebccbbf83 100644
--- a/packages/@angular/cli/models/webpack-configs/browser.ts
+++ b/packages/@angular/cli/models/webpack-configs/browser.ts
@@ -8,14 +8,11 @@ import { packageChunkSort } from '../../utilities/package-chunk-sort';
 import { BaseHrefWebpackPlugin } from '../../lib/base-href-webpack';
 import { extraEntryParser, lazyChunksFilter } from './utils';
 import { WebpackConfigOptions } from '../webpack-config';
-import { requireProjectModule } from '../../utilities/require-project-module';
 
 
 export function getBrowserConfig(wco: WebpackConfigOptions) {
   const { projectRoot, buildOptions, appConfig } = wco;
 
-  const projectTs = requireProjectModule(projectRoot, 'typescript');
-
   const appRoot = path.resolve(projectRoot, appConfig.root);
 
   let extraPlugins: any[] = [];
@@ -80,13 +77,10 @@ export function getBrowserConfig(wco: WebpackConfigOptions) {
     }));
   }
 
-  const supportES2015 = wco.tsConfig.options.target !== projectTs.ScriptTarget.ES3
-                     && wco.tsConfig.options.target !== projectTs.ScriptTarget.ES5;
-
   return {
     resolve: {
       mainFields: [
-        ...(supportES2015 ? ['es2015'] : []),
+        ...(wco.supportES2015 ? ['es2015'] : []),
         'browser', 'module', 'main'
       ]
     },
diff --git a/packages/@angular/cli/models/webpack-configs/common.ts b/packages/@angular/cli/models/webpack-configs/common.ts
index aa51438e8caf..8c2111b25516 100644
--- a/packages/@angular/cli/models/webpack-configs/common.ts
+++ b/packages/@angular/cli/models/webpack-configs/common.ts
@@ -30,8 +30,6 @@ export function getCommonConfig(wco: WebpackConfigOptions) {
   const appRoot = path.resolve(projectRoot, appConfig.root);
   const nodeModules = path.resolve(projectRoot, 'node_modules');
 
-  const projectTs = requireProjectModule(projectRoot, 'typescript');
-
   let extraPlugins: any[] = [];
   let extraRules: any[] = [];
   let entryPoints: { [key: string]: string[] } = {};
@@ -159,18 +157,11 @@ export function getCommonConfig(wco: WebpackConfigOptions) {
     extraPlugins.push(new NamedLazyChunksWebpackPlugin());
   }
 
-  // Read the tsconfig to determine if we should prefer ES2015 modules.
-
   // Load rxjs path aliases.
   // https://github.com/ReactiveX/rxjs/blob/master/doc/lettable-operators.md#build-and-treeshaking
-
-  const supportES2015 =
-    wco.tsConfig.options.target !== projectTs.ScriptTarget.ES3 &&
-    wco.tsConfig.options.target !== projectTs.ScriptTarget.ES5;
-
   let alias = {};
   try {
-    const rxjsPathMappingImport = supportES2015
+    const rxjsPathMappingImport = wco.supportES2015
       ? 'rxjs/_esm2015/path-mapping'
       : 'rxjs/_esm5/path-mapping';
     const rxPaths = requireProjectModule(projectRoot, rxjsPathMappingImport);
diff --git a/packages/@angular/cli/models/webpack-configs/production.ts b/packages/@angular/cli/models/webpack-configs/production.ts
index 7666b9813911..feab13846220 100644
--- a/packages/@angular/cli/models/webpack-configs/production.ts
+++ b/packages/@angular/cli/models/webpack-configs/production.ts
@@ -8,8 +8,6 @@ import { PurifyPlugin } from '@angular-devkit/build-optimizer';
 import { StaticAssetPlugin } from '../../plugins/static-asset';
 import { GlobCopyWebpackPlugin } from '../../plugins/glob-copy-webpack-plugin';
 import { WebpackConfigOptions } from '../webpack-config';
-import { readTsconfig } from '../../utilities/read-tsconfig';
-import { requireProjectModule } from '../../utilities/require-project-module';
 
 const UglifyJSPlugin = require('uglifyjs-webpack-plugin');
 
@@ -24,8 +22,6 @@ const UglifyJSPlugin = require('uglifyjs-webpack-plugin');
 export function getProdConfig(wco: WebpackConfigOptions) {
   const { projectRoot, buildOptions, appConfig } = wco;
 
-  const projectTs = requireProjectModule(projectRoot, 'typescript');
-
   let extraPlugins: any[] = [];
   let entryPoints: { [key: string]: string[] } = {};
 
@@ -123,12 +119,6 @@ export function getProdConfig(wco: WebpackConfigOptions) {
     uglifyCompressOptions.passes = 3;
   }
 
-  // Read the tsconfig to determine if we should apply ES6 uglify.
-  const tsconfigPath = path.resolve(projectRoot, appConfig.root, appConfig.tsconfig);
-  const tsConfig = readTsconfig(tsconfigPath);
-  const supportES2015 = tsConfig.options.target !== projectTs.ScriptTarget.ES3
-    && tsConfig.options.target !== projectTs.ScriptTarget.ES5;
-
   return {
     entry: entryPoints,
     plugins: [
@@ -140,7 +130,7 @@ export function getProdConfig(wco: WebpackConfigOptions) {
       new UglifyJSPlugin({
         sourceMap: buildOptions.sourcemaps,
         uglifyOptions: {
-          ecma: supportES2015 ? 6 : 5,
+          ecma: wco.supportES2015 ? 6 : 5,
           warnings: buildOptions.verbose,
           ie8: false,
           mangle: true,
diff --git a/packages/@angular/cli/models/webpack-configs/server.ts b/packages/@angular/cli/models/webpack-configs/server.ts
index 72b8f35bb59d..dcfcecd0d6bd 100644
--- a/packages/@angular/cli/models/webpack-configs/server.ts
+++ b/packages/@angular/cli/models/webpack-configs/server.ts
@@ -1,5 +1,4 @@
 import { WebpackConfigOptions } from '../webpack-config';
-import { requireProjectModule } from '../../utilities/require-project-module';
 
 /**
  * Returns a partial specific to creating a bundle for node
@@ -7,15 +6,10 @@ import { requireProjectModule } from '../../utilities/require-project-module';
  */
 export function getServerConfig(wco: WebpackConfigOptions) {
 
-  const projectTs = requireProjectModule(wco.projectRoot, 'typescript');
-
-  const supportES2015 = wco.tsConfig.options.target !== projectTs.ScriptTarget.ES3
-                     && wco.tsConfig.options.target !== projectTs.ScriptTarget.ES5;
-
   const config: any = {
     resolve: {
       mainFields: [
-        ...(supportES2015 ? ['es2015'] : []),
+        ...(wco.supportES2015 ? ['es2015'] : []),
         'main', 'module',
       ],
     },