Add support for assert statements with messages to the analyzer.

pkg/analyzer/lib/src/context/context.dart

Index: pkg/analyzer/lib/src/context/context.dart
diff --git a/pkg/analyzer/lib/src/context/context.dart b/pkg/analyzer/lib/src/context/context.dart
index f472ed15a6b59b95b12bfb9b4d2ab29b6bd487c1..c988204e375bd805a40a69dfb1ff035e933f239e 100644
--- a/pkg/analyzer/lib/src/context/context.dart
+++ b/pkg/analyzer/lib/src/context/context.dart
@@ -252,6 +252,7 @@ class AnalysisContextImpl implements InternalAnalysisContext {
         (this._options.hint && !options.hint) ||
         (this._options.lint && !options.lint) ||
         this._options.preserveComments != options.preserveComments ||
+        this._options.enableAssertMessage != options.enableAssertMessage ||
         this._options.enableStrictCallChecks !=
             options.enableStrictCallChecks ||
         this._options.enableSuperMixins != options.enableSuperMixins;
@@ -264,6 +265,7 @@ class AnalysisContextImpl implements InternalAnalysisContext {
     this._options.generateImplicitErrors = options.generateImplicitErrors;
     this._options.generateSdkErrors = options.generateSdkErrors;
     this._options.dart2jsHint = options.dart2jsHint;
+    this._options.enableAssertMessage = options.enableAssertMessage;
     this._options.enableStrictCallChecks = options.enableStrictCallChecks;
     this._options.enableSuperMixins = options.enableSuperMixins;
     this._options.hint = options.hint;