Add support to dart2js for option --enable-asserts.

pkg/compiler/lib/src/commandline_options.dart

Index: pkg/compiler/lib/src/commandline_options.dart
diff --git a/pkg/compiler/lib/src/commandline_options.dart b/pkg/compiler/lib/src/commandline_options.dart
index dd0a01ee0af9927c88c6ce39a6584257592c9248..4852127392161658b11d335a5ebcb23e6cc15349 100644
--- a/pkg/compiler/lib/src/commandline_options.dart
+++ b/pkg/compiler/lib/src/commandline_options.dart
@@ -20,6 +20,7 @@ class Flags {
   static const String dumpInfo = '--dump-info';
   static const String enableAssertMessage = '--assert-message';
   static const String enableCheckedMode = '--enable-checked-mode';
+  static const String enableAssert = '--enable-assert';

[sra1, 2017/05/15 20:18:43]

The VM has undocumented options:

    --enable-asserts       // ends in 's'.
    --enable-type-checks

If we proceed with this CL, we should support consistent command-line options
across all compilers.

[Johnni Winther, 2017/05/16 07:38:49]

Acknowledged.

[eernst, 2017/05/23 17:09:55]

Done.

   static const String enableDiagnosticColors = '--enable-diagnostic-colors';
   static const String enableExperimentalMirrors =
       '--enable-experimental-mirrors';