diff --git a/packages/@angular/cli/commands/build.ts b/packages/@angular/cli/commands/build.ts
index 9630918f2e..d3778a8ab9 100644
--- a/packages/@angular/cli/commands/build.ts
+++ b/packages/@angular/cli/commands/build.ts
@@ -220,6 +220,11 @@ const BuildCommand = Command.extend({
       commandOptions.vendorChunk = !commandOptions.buildOptimizer;
     }
 
+    // Force commonjs module format for TS on dev watch builds.
+    if (commandOptions.target === 'development' && commandOptions.watch === true) {
+      commandOptions.forceTsCommonjs = true;
+    }
+
     const BuildTask = require('../tasks/build').default;
 
     const buildTask = new BuildTask({
diff --git a/packages/@angular/cli/commands/serve.ts b/packages/@angular/cli/commands/serve.ts
index 384f6ef054..b4edd15d42 100644
--- a/packages/@angular/cli/commands/serve.ts
+++ b/packages/@angular/cli/commands/serve.ts
@@ -135,6 +135,11 @@ const ServeCommand = Command.extend({
       commandOptions.vendorChunk = !commandOptions.buildOptimizer;
     }
 
+    // Force commonjs module format for TS on dev builds.
+    if (commandOptions.target === 'development') {
+      commandOptions.forceTsCommonjs = true;
+    }
+
     // Default evalSourcemaps to true for serve. This makes rebuilds faster.
     commandOptions.evalSourcemaps = true;
 
diff --git a/packages/@angular/cli/commands/test.ts b/packages/@angular/cli/commands/test.ts
index a585b20b7d..40fa3ebaa6 100644
--- a/packages/@angular/cli/commands/test.ts
+++ b/packages/@angular/cli/commands/test.ts
@@ -24,6 +24,7 @@ export interface TestOptions {
   environment?: string;
   app?: string;
   preserveSymlinks?: boolean;
+  forceTsCommonjs?: boolean;
 }
 
 
@@ -134,6 +135,13 @@ const TestCommand = Command.extend({
       // if not watching ensure karma is doing a single run
       commandOptions.singleRun = true;
     }
+
+    // Don't force commonjs for code coverage builds, some setups need es2015 for it.
+    // https://github.com/angular/angular-cli/issues/5526
+    if (!commandOptions.codeCoverage) {
+      commandOptions.forceTsCommonjs = true;
+    }
+
     return testTask.run(commandOptions);
   }
 });
diff --git a/packages/@angular/cli/models/build-options.ts b/packages/@angular/cli/models/build-options.ts
index 4826d5eebe..ff605bb56d 100644
--- a/packages/@angular/cli/models/build-options.ts
+++ b/packages/@angular/cli/models/build-options.ts
@@ -30,4 +30,5 @@ export interface BuildOptions {
   buildOptimizer?: boolean;
   namedChunks?: boolean;
   subresourceIntegrity?: boolean;
+  forceTsCommonjs?: boolean;
 }
diff --git a/packages/@angular/cli/models/webpack-configs/typescript.ts b/packages/@angular/cli/models/webpack-configs/typescript.ts
index e00e212d35..780640b123 100644
--- a/packages/@angular/cli/models/webpack-configs/typescript.ts
+++ b/packages/@angular/cli/models/webpack-configs/typescript.ts
@@ -21,10 +21,17 @@ const webpackLoader: string = g['angularCliIsLocal']
 function _createAotPlugin(wco: WebpackConfigOptions, options: any) {
   const { appConfig, projectRoot, buildOptions } = wco;
   options.compilerOptions = options.compilerOptions || {};
+
   if (wco.buildOptions.preserveSymlinks) {
     options.compilerOptions.preserveSymlinks = true;
   }
 
+  // Forcing commonjs seems to drastically improve rebuild speeds on webpack.
+  // Dev builds on watch mode will set this option to true.
+  if (wco.buildOptions.forceTsCommonjs) {
+    options.compilerOptions.module = 'commonjs';
+  }
+
   // Read the environment, and set it in the compiler host.
   let hostReplacementPaths: any = {};
   // process environment file replacement
diff --git a/packages/@angular/cli/tasks/test.ts b/packages/@angular/cli/tasks/test.ts
index 42d647d78d..d9e75985bf 100644
--- a/packages/@angular/cli/tasks/test.ts
+++ b/packages/@angular/cli/tasks/test.ts
@@ -44,6 +44,7 @@ export default Task.extend({
         poll: options.poll,
         environment: options.environment,
         preserveSymlinks: options.preserveSymlinks,
+        forceTsCommonjs: options.forceTsCommonjs,
         app: options.app
       };