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..4fc1b2fed15a1c26c7938367dbbe4d01f07d5843 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
@@ -41,6 +41,13 @@ throwAssertionError() => JS('', '''(() => {
   $throw_(new $AssertionError());
 })()''');
 
+throwAssertionErrorWithMessage(message) => JS(

[Jennifer Messerly, 2016/12/12 22:36:02]

this can be removed per earlier suggestion

[Bob Nystrom, 2016/12/13 19:29:51]

Done.

+    '',
+    '''(() => {
+  debugger;
+  $throw_(new $AssertionErrorWithMessage($message));
+})()''');
+
 throwNullValueError() => JS('', '''(() => {
   // TODO(vsm): Per spec, we should throw an NSM here.  Technically, we ought
   // to thread through method info, but that uglifies the code and can't