Add command-line parameter enabling assert messages.

pkg/compiler/lib/src/apiimpl.dart

Index: pkg/compiler/lib/src/apiimpl.dart
diff --git a/pkg/compiler/lib/src/apiimpl.dart b/pkg/compiler/lib/src/apiimpl.dart
index d3ca1bc4b0aaa49ced8033ba3f3e040091f1eaef..4f079db61a4483b8fdf64e784d7937c87434dcd2 100644
--- a/pkg/compiler/lib/src/apiimpl.dart
+++ b/pkg/compiler/lib/src/apiimpl.dart
@@ -118,7 +118,8 @@ class Compiler extends leg.Compiler {
                 hasOption(options, '--generate-code-with-compile-time-errors'),
             testMode: hasOption(options, '--test-mode'),
             allowNativeExtensions:
-                hasOption(options, '--allow-native-extensions')) {
+                hasOption(options, '--allow-native-extensions'),
+            allowAssertMessage: hasOption(options, '--enable-assert-message')) {
     tasks.addAll([
         userHandlerTask = new GenericTask('Diagnostic handler', this),
         userProviderTask = new GenericTask('Input provider', this),