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

tools/testing/dart/compiler_configuration.dart

Index: tools/testing/dart/compiler_configuration.dart
diff --git a/tools/testing/dart/compiler_configuration.dart b/tools/testing/dart/compiler_configuration.dart
index fffd68135bc7edc7ebd58795d48e7023a48d7144..30121418ebbf612ea81a68bb46fb5c66dc64b375 100644
--- a/tools/testing/dart/compiler_configuration.dart
+++ b/tools/testing/dart/compiler_configuration.dart
@@ -67,6 +67,7 @@ abstract class CompilerConfiguration {
     bool hotReload = configuration['hot_reload'];
     bool hotReloadRollback = configuration['hot_reload_rollback'];
     bool useFastStartup = configuration['fast_startup'];
+    bool useEnableAsserts = configuration['enable_asserts'];
     bool useKernelInDart2js = configuration['dart2js_with_kernel'];
 
     switch (compiler) {
@@ -86,6 +87,7 @@ abstract class CompilerConfiguration {
             useSdk: useSdk,
             isCsp: isCsp,
             useFastStartup: useFastStartup,
+            useEnableAsserts: useEnableAsserts,
             useKernel: useKernelInDart2js,
             extraDart2jsOptions:
                 TestUtils.getExtraOptions(configuration, 'dart2js_options'));
@@ -539,6 +541,7 @@ class Dart2jsCompilerConfiguration extends Dart2xCompilerConfiguration {
   final bool isCsp;
   final bool useCps;
   final bool useFastStartup;
+  final bool useEnableAsserts;
   final bool useKernel;
   final List<String> extraDart2jsOptions;
   // We cache the extended environment to save memory.
@@ -553,6 +556,7 @@ class Dart2jsCompilerConfiguration extends Dart2xCompilerConfiguration {
       bool this.useCps,
       bool this.isCsp,
       bool this.useFastStartup,
+      bool this.useEnableAsserts,
       this.useKernel,
       this.extraDart2jsOptions})
       : super('dart2js',