Add command-line parameter enabling assert messages.

tests/compiler/dart2js/parser_helper.dart

Index: tests/compiler/dart2js/parser_helper.dart
diff --git a/tests/compiler/dart2js/parser_helper.dart b/tests/compiler/dart2js/parser_helper.dart
index c2d3bdfed0ac1d0c73c333b46c39a165dd837c2e..cc0486b7dcfb4f31dc35059a5abccb0fa7119525 100644
--- a/tests/compiler/dart2js/parser_helper.dart
+++ b/tests/compiler/dart2js/parser_helper.dart
@@ -125,7 +125,9 @@ Link<Element> parseUnit(String text, Compiler compiler,
   var unit = new CompilationUnitElementX(script, library);
   int id = 0;
   ElementListener listener = new ElementListener(compiler, unit, () => id++);
-  PartialParser parser = new PartialParser(listener);
+  PartialParser parser =
+      new PartialParser(listener,
+                        allowAssertMessage: compiler.allowAssertMessage);
   compiler.withCurrentElement(unit, () => parser.parseUnit(tokens));
   return unit.localMembers;
 }