fix(@angular/cli): use RxJS ES2015 path alias when targeting ES2015

Author: sarunint

packages/@angular/cli/models/webpack-configs/common.ts

@@ -30,6 +30,8 @@ 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[] } = {};
@@ -161,9 +163,16 @@ export function getCommonConfig(wco: WebpackConfigOptions) {
 
   // 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 = 'rxjs/_esm5/path-mapping';
+    const rxjsPathMappingImport = supportES2015
+      ? 'rxjs/_esm2015/path-mapping'
+      : 'rxjs/_esm5/path-mapping';
     const rxPaths = requireProjectModule(projectRoot, rxjsPathMappingImport);
     alias = rxPaths(nodeModules);
   } catch (e) { }