add --assert-message flag back

pkg/compiler/lib/src/options.dart

Index: pkg/compiler/lib/src/options.dart
diff --git a/pkg/compiler/lib/src/options.dart b/pkg/compiler/lib/src/options.dart
index 777f4b00d9300ca82b1d8c146621af9e9c33b3c7..1c257dfba9867d8f32a0849e9887064262c5ee35 100644
--- a/pkg/compiler/lib/src/options.dart
+++ b/pkg/compiler/lib/src/options.dart
@@ -129,6 +129,10 @@ class CompilerOptions implements DiagnosticOptions {
   /// code for each function.
   final bool dumpInfo;
 
+  /// Whether we allow passing an extra argument to `assert`, containing a
+  /// reason for why an assertion fails. (experimental)
+  final bool enableAssertMessage;
+
   /// Whether the user specified a flag to allow the use of dart:mirrors. This
   /// silences a warning produced by the compiler.
   final bool enableExperimentalMirrors;
@@ -328,6 +332,7 @@ class CompilerOptions implements DiagnosticOptions {
       bool disableInlining: false,
       bool disableTypeInference: false,
       bool dumpInfo: false,
+      bool enableAssertMessage: true,
       bool enableExperimentalMirrors: false,
       bool enableMinification: false,
       bool enableNativeLiveTypeAnalysis: true,
@@ -397,6 +402,7 @@ class CompilerOptions implements DiagnosticOptions {
         disableInlining: disableInlining || hasIncrementalSupport,
         disableTypeInference: disableTypeInference,
         dumpInfo: dumpInfo,
+        enableAssertMessage: enableAssertMessage,
         enableExperimentalMirrors: enableExperimentalMirrors,
         enableMinification: enableMinification,
         enableNativeLiveTypeAnalysis: enableNativeLiveTypeAnalysis,
@@ -445,6 +451,7 @@ class CompilerOptions implements DiagnosticOptions {
       this.disableInlining: false,
       this.disableTypeInference: false,
       this.dumpInfo: false,
+      this.enableAssertMessage: true,
       this.enableExperimentalMirrors: false,
       this.enableMinification: false,
       this.enableNativeLiveTypeAnalysis: false,
@@ -500,6 +507,7 @@ class CompilerOptions implements DiagnosticOptions {
       disableInlining,
       disableTypeInference,
       dumpInfo,
+      enableAssertMessage,
       enableExperimentalMirrors,
       enableMinification,
       enableNativeLiveTypeAnalysis,
@@ -556,6 +564,7 @@ class CompilerOptions implements DiagnosticOptions {
         disableTypeInference:
             disableTypeInference ?? options.disableTypeInference,
         dumpInfo: dumpInfo ?? options.dumpInfo,
+        enableAssertMessage: enableAssertMessage ?? options.enableAssertMessage,
         enableExperimentalMirrors:
             enableExperimentalMirrors ?? options.enableExperimentalMirrors,
         enableMinification: enableMinification ?? options.enableMinification,