Implement messages in assert() in dev_compiler.

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/errors.dart

Index: pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/errors.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/errors.dart b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/errors.dart
index 244a603a082c32421564a9e94fe32254c4de9285..0f8d0b407f7c1577156d7aa6c405b79e426da9c4 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/errors.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/errors.dart
@@ -36,9 +36,12 @@ throwUnimplementedError(message) => JS('', '''(() => {
   $throw_(new $UnimplementedError($message));
 })()''');
 
-throwAssertionError() => JS('', '''(() => {
+throwAssertionError([message]) => JS('', '''(() => {
   debugger;
-  $throw_(new $AssertionError());
+  let error = $message != null
+        ? new $AssertionErrorWithMessage($message())
+        : new $AssertionError();
+  $throw_(error);
 })()''');
 
 throwNullValueError() => JS('', '''(() => {